Remove 3.5 and xenial support (SC-711) (#1167)

Includes:
 - Update tox.ini and .travis.yml accordingly
 - Cleanup tox.ini with new tox syntax and cloud-init dependencies
 - Update documentation accordingly
 - Replace/remove xenial references where additional testing isn't required
 - Remove xenial checks in integration tests
 - Replace yield_fixture with fixture in pytest tests

Sections of code commented with lines like "Remove when Xenial is no
longer supported" still exist as they're require additional testing.

1 files changed, 20 insertions, 21 deletions

diff --git a/.travis.yml b/.travis.yml
index 2351246b..208bed23 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -75,17 +75,17 @@ matrix:
             - cp /usr/share/doc/sbuild/examples/example.sbuildrc /home/$USER/.sbuildrc
           script:
             # Ubuntu LTS: Build
-            - ./packages/bddeb -S -d --release xenial
+            - ./packages/bddeb -S -d --release bionic
             - |
                 needs_caching=false
-                if [ -e "$TRAVIS_BUILD_DIR/chroots/xenial-amd64.tar" ]; then
+                if [ -e "$TRAVIS_BUILD_DIR/chroots/bionic-amd64.tar" ]; then
                     # If we have a cached chroot, move it into place
-                    sudo mkdir -p /var/lib/schroot/chroots/xenial-amd64
-                    sudo tar --sparse --xattrs --preserve-permissions --numeric-owner -xf "$TRAVIS_BUILD_DIR/chroots/xenial-amd64.tar" -C /var/lib/schroot/chroots/xenial-amd64
+                    sudo mkdir -p /var/lib/schroot/chroots/bionic-amd64
+                    sudo tar --sparse --xattrs --preserve-permissions --numeric-owner -xf "$TRAVIS_BUILD_DIR/chroots/bionic-amd64.tar" -C /var/lib/schroot/chroots/bionic-amd64
                     # Write its configuration
-                    cat > sbuild-xenial-amd64 << EOM
-                [xenial-amd64]
-                description=xenial-amd64
+                    cat > sbuild-bionic-amd64 << EOM
+                [bionic-amd64]
+                description=bionic-amd64
                 groups=sbuild,root,admin
                 root-groups=sbuild,root,admin
                 # Uncomment these lines to allow members of these groups to access
@@ -95,20 +95,20 @@ matrix:
                 type=directory
                 profile=sbuild
                 union-type=overlay
-                directory=/var/lib/schroot/chroots/xenial-amd64
+                directory=/var/lib/schroot/chroots/bionic-amd64
                 EOM
-                    sudo mv sbuild-xenial-amd64 /etc/schroot/chroot.d/
-                    sudo chown root /etc/schroot/chroot.d/sbuild-xenial-amd64
+                    sudo mv sbuild-bionic-amd64 /etc/schroot/chroot.d/
+                    sudo chown root /etc/schroot/chroot.d/sbuild-bionic-amd64
                     # And ensure it's up-to-date.
-                    before_pkgs="$(sudo schroot -c source:xenial-amd64 -d / dpkg -l | sha256sum)"
-                    sudo schroot -c source:xenial-amd64 -d / -- sh -c "apt-get update && apt-get -qqy upgrade"
-                    after_pkgs=$(sudo schroot -c source:xenial-amd64 -d / dpkg -l | sha256sum)
+                    before_pkgs="$(sudo schroot -c source:bionic-amd64 -d / dpkg -l | sha256sum)"
+                    sudo schroot -c source:bionic-amd64 -d / -- sh -c "apt-get update && apt-get -qqy upgrade"
+                    after_pkgs=$(sudo schroot -c source:bionic-amd64 -d / dpkg -l | sha256sum)
                     if [ "$before_pkgs" != "$after_pkgs" ]; then
                         needs_caching=true
                     fi
                 else
                     # Otherwise, create the chroot
-                    sudo -E su $USER -c 'mk-sbuild xenial'
+                    sudo -E su $USER -c 'mk-sbuild bionic'
                     needs_caching=true
                 fi
                 # If there are changes to the schroot (or it's entirely new),
@@ -116,19 +116,19 @@ matrix:
                 # move it into the cached dir; no need to compress it because
                 # Travis will do that anyway
                 if [ "$needs_caching" = "true" ]; then
-                    sudo tar --sparse --xattrs --xattrs-include=* -cf "$TRAVIS_BUILD_DIR/chroots/xenial-amd64.tar" -C /var/lib/schroot/chroots/xenial-amd64 .
+                    sudo tar --sparse --xattrs --xattrs-include=* -cf "$TRAVIS_BUILD_DIR/chroots/bionic-amd64.tar" -C /var/lib/schroot/chroots/bionic-amd64 .
                 fi
             # Use sudo to get a new shell where we're in the sbuild group
             # Don't run integration tests when build fails
             - |
-                sudo -E su $USER -c 'DEB_BUILD_OPTIONS=nocheck sbuild --nolog --no-run-lintian --no-run-autopkgtest --verbose --dist=xenial cloud-init_*.dsc' &&
+                sudo -E su $USER -c 'DEB_BUILD_OPTIONS=nocheck sbuild --nolog --no-run-lintian --no-run-autopkgtest --verbose --dist=bionic cloud-init_*.dsc' &&
                   ssh-keygen -P "" -q -f ~/.ssh/id_rsa                                                         &&
                   sg lxd -c 'CLOUD_INIT_CLOUD_INIT_SOURCE="$(ls *.deb)" tox -e integration-tests-ci'
-        - python: 3.5
+        - python: 3.6
           env:
-              TOXENV=xenial
-              PYTEST_ADDOPTS=-v  # List all tests run by pytest
-          dist: xenial
+            TOXENV=lowest-supported
+            PYTEST_ADDOPTS=-v  # List all tests run by pytest
+          dist: bionic
         - python: 3.6
           env: TOXENV=flake8
         - python: 3.6
@@ -145,4 +145,3 @@ matrix:
         - python: 3.9
         - python: 3.8
         - python: 3.7
-        - python: 3.5