Drop support for Python 3.5

Running repo with Python 3.5 fails due to the use of the encoding
parameter to subprocess.run(). There are also f-strings being used in
some of the tests.

This drops support for these systems:
* Ubuntu Xenial: released Apr 2016, EOS Apr 2021, EOL Apr 2024
* Debian Stretch: released Jun 2017, EOL Jun 2022

So the minimum required distros now are:
* Ubuntu Bionic: released Apr 2018 w/Python 3.6
* Debian Buster: released Jul 2019 w/Python 3.7

Change-Id: I1144f7ab6f882b10cac0131982df081fe4ac44f9
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/303363
Reviewed-by: Mike Frysinger <vapier@google.com>
Tested-by: Peter Kjellerstedt <peter.kjellerstedt@axis.com>
diff --git a/.github/workflows/test-ci.yml b/.github/workflows/test-ci.yml
index ec6f379..1988185 100644
--- a/.github/workflows/test-ci.yml
+++ b/.github/workflows/test-ci.yml
@@ -14,7 +14,7 @@
       fail-fast: false
       matrix:
         os: [ubuntu-latest, macos-latest, windows-latest]
-        python-version: [3.5, 3.6, 3.7, 3.8, 3.9]
+        python-version: [3.6, 3.7, 3.8, 3.9]
     runs-on: ${{ matrix.os }}
 
     steps: