Remove the use of tox-gh-actions

In this package, the GH actions always run a consistent set of tox
environments only.  Using tox-gh-actions has no real advantage over
running a dedicated tox env (i.e. plain `py`) and requires duplicating
all the environment declarations.

Author: mgorny

.github/workflows/tests.yml

@@ -23,6 +23,6 @@ jobs:
     - name: Install dependencies
       run: |
         python -m pip install --upgrade pip
-        pip install tox tox-gh-actions
+        pip install tox
     - name: Test with tox
-      run: tox -vv
+      run: tox -vv -e py

tox.ini

@@ -6,15 +6,6 @@ isolated_build = True
 [tox:hudson]
 downloadcache = {toxworkdir}/_download
 
-[gh-actions]
-python =
-    3.5: py35
-    3.6: py36
-    3.7: py37
-    3.8: py38
-    3.9: py39
-    3.10: py310
-
 [testenv]
 description = run test suite under {basepython}
 deps =