workflow: remove python 2.7 testing

Python 2.7 is long obsolete, deprecated and not supported any more.
Remove it from the tests. This test job is especially supposed to run on
ubuntu-20.04, which doesn't have Python 2.7 any more.

Without this patch, this workflow fails with:

  Warning: The support for python 2.7 will be removed on June 19. Related issue: actions/setup-python#672
  Version 2.7 was not found in the local cache
  Error: The version '2.7' with architecture 'x64' was not found for Ubuntu 20.04.
  The list of all available versions can be found here: https://raw.githubusercontent.com/actions/python-versions/main/versions-manifest.json

.github/workflows/main.yml

@@ -16,7 +16,6 @@ jobs:
       fail-fast: false
       matrix:
         python-version:
-        - '2.7'
         - '3.7'
         - '3.8'
         - '3.9'