Moved flake8, shell and documentation tests to Github Action (#17328)

* Move flake8 tests on Github Actions

* Move shell test to Github Actions

* Moved documentation build to Github Action

* Don't run coverage on Python 2.6

Since we get connection errors consistently on Travis
when trying to upload coverage results for Python 2.6,
avoid computing coverage entirely to speed-up tests.

lib/spack/spack/test/llnl/util/lock.py

@@ -1143,8 +1143,6 @@ def read():
                     assert vals['read'] == 1
 
 
-@pytest.mark.skipif('macos' in os.environ.get('GITHUB_WORKFLOW', ''),
-                    reason="Skip failing test for GA on MacOS")
 def test_lock_debug_output(lock_path):
     host = socket.getfqdn()