salt: Missing dependency 'distro' in Python 3.8 or higher

Description of Issue

Python’s original platform.linux_distribution function will be removed in Python 3.8. In the #50822 and #51541, this is fixed by using the distro library on an ImportError in both salt/version.py and salt/grains/core.py. The distro library isn’t mentioned as a dependency and therefore the install of salt>=2017.7.0 will result in an error.

Snippet importing the distro library:

# linux_distribution deprecated in py3.7
try:
    from platform import linux_distribution
    from platform import linux_distribution as _deprecated_linux_distribution

    def linux_distribution(**kwargs):
        with warnings.catch_warnings():
            warnings.simplefilter("ignore")
            return _deprecated_linux_distribution(**kwargs)
except ImportError:
    from distro import linux_distribution # <-- used but not installed by default

Setup

• python>=3.8
• salt>=2017.7.0

Steps to Reproduce Issue

The issue can be reproduced by installing Salt (via pip) in a Docker Python 3.8 container, by running the following command:

docker run -it python:3.8 /bin/bash -c "pip install salt"

This results in the following error message:

Collecting salt
    ERROR: Command errored out with exit status 1:
     command: /usr/local/bin/python -c 'import sys, setuptools, tokenize; sys.argv[0] = '"'"'/tmp/pip-install-zt9okj4a/salt/setup.py'"'"'; __file__='"'"'/tmp/pip-install-zt9okj4a/salt/setup.py'"'"';f=getattr(tokenize, '"'"'open'"'"', open)(__file__);code=f.read().replace('"'"'\r\n'"'"', '"'"'\n'"'"');f.close();exec(compile(code, __file__, '"'"'exec'"'"'))' egg_info --egg-base /tmp/pip-install-zt9okj4a/salt/pip-egg-info
         cwd: /tmp/pip-install-zt9okj4a/salt/
    Complete output (14 lines):
    Traceback (most recent call last):
      File "/tmp/pip-install-zt9okj4a/salt/salt/version.py", line 15, in <module>
        from platform import linux_distribution as _deprecated_linux_distribution
    ImportError: cannot import name 'linux_distribution' from 'platform' (/usr/local/lib/python3.8/platform.py)
    
    During handling of the above exception, another exception occurred:
    
    Traceback (most recent call last):
      File "<string>", line 1, in <module>
      File "/tmp/pip-install-zt9okj4a/salt/setup.py", line 137, in <module>
        exec(compile(open(SALT_VERSION).read(), SALT_VERSION, 'exec'))
      File "/tmp/pip-install-zt9okj4a/salt/salt/version.py", line 22, in <module>
        from distro import linux_distribution
    ModuleNotFoundError: No module named 'distro'
    ----------------------------------------
ERROR: Command errored out with exit status 1: python setup.py egg_info Check the logs for full command output.

Affected Versions

Quick test shows that Salt 2017.7.0 and higher are affected when being installed on Python 3.8.