stop testing Python 2.7

A recent change broke `repo version` on Python 2.7.  Rather than
fix it, lets drop Python 2.7 support so it can slowly rot.

Bug: https://crbug.com/gerrit/10418
Change-Id: I5c6e3d18e4a193b0a978062c23f7cea392e95d0f
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/259155
Reviewed-by: David Pursehouse <david.pursehouse@gmail.com>
Tested-by: Mike Frysinger <vapier@google.com>

.github/workflows/test-ci.yml

@@ -14,10 +14,7 @@ jobs:
       fail-fast: false
       matrix:
         os: [ubuntu-latest, macos-latest, windows-latest]
-        python-version: [2.7, 3.6, 3.7, 3.8]
-        exclude:
-          - os: windows-latest
-            python-version: 2.7
+        python-version: [3.6, 3.7, 3.8]
     runs-on: ${{ matrix.os }}
 
     steps:

tox.ini

@@ -15,11 +15,10 @@
 # https://tox.readthedocs.io/
 
 [tox]
-envlist = py27, py36, py37, py38
+envlist = py36, py37, py38
 
 [gh-actions]
 python =
-    2.7: py27
     3.6: py36
     3.7: py37
     3.8: py38
@@ -31,8 +30,3 @@ setenv =
     GIT_AUTHOR_NAME = Repo test author
     GIT_COMMITTER_NAME = Repo test committer
     EMAIL = repo@gerrit.nodomain
-
-[testenv:py27]
-deps =
-    mock
-    pytest