chore: Remove support for Python 3.6 (#1354)

* chore: Remove support for Python 3.6

* chore: Refresh system test creds.

* Revert "chore: Remove support for Python 3.6"

This reverts commit 3bfd7ba.

* Add deprecation notice for 3.6 and 3.7.

* chore: Refresh system test creds.

* Revert "Revert "chore: Remove support for Python 3.6""

This reverts commit c9f006b.

* Revert "Add deprecation notice for 3.6 and 3.7."

This reverts commit fb6b619.

* Bump mypy Python version.

* PR feedback.

Author: clundin25

.kokoro/samples/python3.6/common.cfg

@@ -19,7 +19,7 @@ A few notes on making changes to ``google-auth-library-python``.
   using ``nox -s docgen``.
 
 - The change must work fully on the following CPython versions:
-  3.6, 3.7, 3.8, 3.9, 3.10 across macOS, Linux, and Windows.
+  3.7, 3.8, 3.9, 3.10 across macOS, Linux, and Windows.
 
 - The codebase *must* have 100% test statement coverage after each commit.
   You can test coverage via ``nox -e cover``.

.kokoro/samples/python3.6/continuous.cfg

@@ -35,7 +35,7 @@ Note that the extras pyopenssl and enterprise_cert should not be used together b
 
 Supported Python Versions
 ^^^^^^^^^^^^^^^^^^^^^^^^^
-Python >= 3.6
+Python >= 3.7
 
 Unsupported Python Versions
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^
@@ -45,6 +45,9 @@ Unsupported Python Versions
 - Python 3.5:   The last version of this library with support for Python 3.5
   was `google.auth == 1.23.0`.
 
+- Python 3.6:   The last version of this library with support for Python 3.6
+  was `google.auth == 2.22.0`.
+
 Documentation
 -------------
 

.kokoro/samples/python3.6/periodic-head.cfg

@@ -193,7 +193,7 @@ def retrieve_subject_token(self, request):
 
         if not _helpers.is_python_3():
             raise exceptions.RefreshError(
-                "Pluggable auth is only supported for python 3.6+"
+                "Pluggable auth is only supported for python 3.7+"
             )
 
         # Inject env vars.
@@ -255,7 +255,7 @@ def revoke(self, request):
 
         if not _helpers.is_python_3():
             raise exceptions.RefreshError(
-                "Pluggable auth is only supported for python 3.6+"
+                "Pluggable auth is only supported for python 3.7+"
             )
 
         # Inject variables

.kokoro/samples/python3.6/periodic.cfg

@@ -1,3 +1,3 @@
 [mypy]
-python_version = 3.6
+python_version = 3.7
 namespace_packages = True

.kokoro/samples/python3.6/presubmit.cfg

@@ -84,7 +84,7 @@ def mypy(session):
     session.run("mypy", "-p", "google", "-p", "tests", "-p", "tests_async")
 
 
-@nox.session(python=["3.6", "3.7", "3.8", "3.9", "3.10", "3.11"])
+@nox.session(python=["3.7", "3.8", "3.9", "3.10", "3.11"])
 def unit(session):
     constraints_path = str(
         CURRENT_DIRECTORY / "testing" / f"constraints-{session.python}.txt"