Revert "Remove py 3.6 support and update jobs"

This reverts commit fe0e5324d4.

Reason for revert: Python3.6 is still being used on Centos 8 based
platforms.

This is a partial revert, since the py36 job is currently failing, it
will be restored in a follow-up patch.

Change-Id: Idc0373f9a639cd66925543376fb1e2e3398666da

releasenotes/notes/python-3.8-b401ad4f2bbbc99f.yaml

@@ -1,3 +0,0 @@
----
-upgrade:
-  - This release removes support for Python releases before 3.8

setup.cfg

@@ -12,10 +12,12 @@ classifier =
     License :: OSI Approved :: Apache Software License
     Operating System :: POSIX :: Linux
     Programming Language :: Python :: 3
+    Programming Language :: Python :: 3.6
+    Programming Language :: Python :: 3.7
     Programming Language :: Python :: 3.8
     Programming Language :: Python :: 3.9
     Environment :: Console
-python_requires = >=3.8
+python_requires = >=3.6
 
 [files]
 packages =