Cleanup Python versions used for GitHub Actions

GitHub Actions no longer supports Python versions prior to 3.7, including 2.7.

Also, `docs/index.rst` has declared 3.6 to be the minimum supported version
since 06b68d6, and python-fitparse has not compiled with Python 2.7 since
f9ad987 (which uses Python 3.x-only
syntax).

.github/workflows/test.yml

@@ -11,7 +11,7 @@ jobs:
  runs-on: ubuntu-latest
  strategy:
  matrix:
- python-version: ['2.7', '3.5', '3.6', '3.x', 'pypy2', 'pypy3']
+ python-version: ['3.7', '3.8', '3.9', '3.10', '3.11', '3.x']
 
  steps:
  - uses: actions/checkout@v2