Python runtimes for Zed are 3.8, 3.9 and optionally 3.10 [1][2],
so tox envs in tox.ini are updated accordingly.

Declare python_requires >=3.8 in setup.cfg
python_requires are declared in many projects during dropping
python 2.7 support. It is good to follow the convention and
it also clarifies our python interpreter requirement.

This commit also cleans up python2.7 related stuffs in setup.py.
Both are related to the supported versions of python,
so I made both changes in a single commit.

[1] https://governance.openstack.org/tc/reference/runtimes/zed.html
[2] https://opendev.org/openstack/openstack-zuul-jobs/src/commit/b740b42fb199d4d695391aaaddf46ca55eab79ac/zuul.d/project-templates.yaml#L542

Change-Id: I4161816fd1fec19a3834cc7d994b3f761e1ab2ce
Closes-Bug: #1960887

Change-Id: Id9d305a3f189294f139402cdba3f93c0904907aa

If 'noqa' is specified all hacking checks are disabled.
It is better to ignore specific rules explicitly rather than
ignoring all. The recent flake8 supports this [1].

[1] http://flake8.pycqa.org/en/latest/user/ignoring-errors.html

Change-Id: I74246adb4d1d2613c2703cf519428b64d00e59bb

Change-Id: I27217dfd6ffd1702cc4ae2b55bfc8db4fe5fb94e

Change-Id: I4dc6485edb7d1639234dec4f6c8ccf42fd9eb074

Change-Id: Ibb4dc8d22f737702cc17661044be0a7845830802

Change-Id: Idc25bccb0147f0f00d07ad0479ba6703c8fb89b5

Change-Id: Ia328c2ff8f3e29e2aff9f3a1a3978dda0fdf3b26

Change-Id: If8e08b4fb3fa32c9e9b6945a6833c97f4b499f1f

Change-Id: I053b84cd2a13f22e654b8fe94c62df60f55f3ec6