Drop py36 and py37 support

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] b740b42fb1/zuul.d/project-templates.yaml (L542)

Change-Id: I4161816fd1fec19a3834cc7d994b3f761e1ab2ce
Closes-Bug: #1960887

.zuul.d/xstatic-master.yaml

@@ -27,7 +27,7 @@
 
 - job:
     name: horizon-tox-py36-xstatic-master
-    parent: openstack-tox-py36
+    parent: openstack-tox-py39
     required-projects: *xstatic-projects
 
 - job:

setup.cfg

@@ -6,6 +6,7 @@ description_file =
 author = OpenStack
 author_email = openstack-discuss@lists.openstack.org
 home_page = https://docs.openstack.org/horizon/latest/
+python_requires = >=3.8
 classifier =
     Development Status :: 5 - Production/Stable
     Environment :: OpenStack
@@ -19,8 +20,6 @@ classifier =
     Programming Language :: Python
     Programming Language :: Python :: 3 :: Only
     Programming Language :: Python :: 3
-    Programming Language :: Python :: 3.6
-    Programming Language :: Python :: 3.7
     Programming Language :: Python :: 3.8
     Programming Language :: Python :: 3.9
     Topic :: Internet :: WWW/HTTP

setup.py

@@ -13,16 +13,8 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
-# THIS FILE IS MANAGED BY THE GLOBAL REQUIREMENTS REPO - DO NOT EDIT
 import setuptools
 
-# In python < 2.7.4, a lazy loading of package `pbr` will break
-# setuptools if some other modules registered functions in `atexit`.
-# solution from: http://bugs.python.org/issue15881#msg170215
-try:
-    import multiprocessing  # noqa
-except ImportError:
-    pass
 
 setuptools.setup(
     setup_requires=['pbr>=2.0.0'],

tox.ini

@@ -1,6 +1,6 @@
 [tox]
 minversion = 3.18.0
-envlist = pep8,py36,py39,releasenotes,npm,py3-dj32
+envlist = pep8,py39,releasenotes,npm,py3-dj32
 skipsdist = True
 # Automatic envs (pyXX) will only use the python version appropriate to that
 # env and ignore basepython inherited from [testenv] if we set
@@ -13,8 +13,9 @@ usedevelop = True
 setenv =
   VIRTUAL_ENV = {envdir}
   PYTHONDONTWRITEBYTECODE = 1
+  # TODO(amotoki): There are three variants of PYTHONWARNINGS. They should be merged.
   PYTHONWARNINGS = once,ignore::PendingDeprecationWarning
-  py{3,36,37}:PYTHONWARNINGS = once,ignore::PendingDeprecationWarning,ignore::ImportWarning:backports
+  py{3,39}:PYTHONWARNINGS = once,ignore::PendingDeprecationWarning,ignore::ImportWarning:backports
   # DeprecationWarning is disabled in dj32 tox env as Django 3.2
   # deprecated Django 2.2 features. This workaround is just to reduce
   # the number of warnings due to this change.