Update python testing as per zed cycle testing runtime

In Zed cycle, we have dropped the python 3.6/3.7[1] testing
and its support. Removing the py36 centos8 job as well as
updating the python classifier also to reflect the same.

[1] https://governance.openstack.org/tc/reference/runtimes/zed.html

Change-Id: I9b5f826863fd913241cb803ef479b890f74fbb6c

releasenotes/notes/drop-python-3-6-and-3-7-f37b9dc6d94620de.yaml

@@ -0,0 +1,5 @@
+---
+upgrade:
+  - |
+    Python 3.6 & 3.7 support has been dropped. The minimum version of Python now
+    supported is Python 3.8.

requirements.txt

@@ -12,7 +12,7 @@ alembic>=0.9.6 # MIT
 botocore>=1.5.1 # Apache-2.0
 castellan>=0.16.0 # Apache-2.0
 eventlet>=0.26.0 # MIT
-Flask>=1.0.2 # BSD
+Flask>=2.1.2 # BSD
 iso8601>=0.1.11 # MIT
 Jinja2>=2.10 # BSD License (3 clause)
 jsonschema>=3.2.0 # MIT

tox.ini

@@ -25,14 +25,6 @@ setenv =
     PACKAGE_NAME=sahara
 commands = {toxinidir}/tools/cover.sh {posargs}
 
-[testenv:debug-py36]
-basepython = python3.6
-commands = oslo_debug_helper -t sahara/tests/unit {posargs}
-
-[testenv:debug-py37]
-basepython = python3.7
-commands = oslo_debug_helper -t sahara/tests/unit {posargs}
-
 [testenv:pep8]
 deps =
   -c{env:TOX_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}