Min version in envlist, when less than the default value for `min-py3-version`, is overridden

When setup-cfg-fmt is run on a library with no arguments and no config except tox.ini with an envlist, then the tool uses a minimum version parsed from tox.ini. For example this test passes:

def test_guess_python_envlist_py38(tmpdir):
    tmpdir.join('tox.ini').write('[tox]\nenvlist = py38\n')
    setup_cfg = tmpdir.join('setup.cfg')
    setup_cfg.write(
        '[metadata]\n'
        'name = pkg\n'
        'version = 1.0\n',
    )

    assert main((str(setup_cfg), ))

    assert setup_cfg.read() == (
        '[metadata]\n'
        'name = pkg\n'
        'version = 1.0\n'
        'classifiers =\n'
        '    Programming Language :: Python :: 3\n'
        '    Programming Language :: Python :: 3 :: Only\n'
        '    Programming Language :: Python :: Implementation :: CPython\n'
        '\n'
        '[options]\n'
        'python_requires = >=3.8\n'
    )

$ pytest -q -k test_guess_python_envlist_py38 tests\setup_cfg_fmt_test.py
.                                                                                                                               [100%]
1 passed, 66 deselected in 0.04s

But when envlist has a lower minimum version (ex: py36) than the default, then python_requires gets set to >= 3.7 incorrectly.

def test_guess_python_envlist_py36(tmpdir):
    tmpdir.join('tox.ini').write('[tox]\nenvlist = py36\n')
    setup_cfg = tmpdir.join('setup.cfg')
    setup_cfg.write(
        '[metadata]\n'
        'name = pkg\n'
        'version = 1.0\n',
    )

    assert main((str(setup_cfg), ))

    assert setup_cfg.read() == (
        '[metadata]\n'
        'name = pkg\n'
        'version = 1.0\n'
        'classifiers =\n'
        '    Programming Language :: Python :: 3\n'
        '    Programming Language :: Python :: 3 :: Only\n'
        '    Programming Language :: Python :: Implementation :: CPython\n'
        '\n'
        '[options]\n'
        'python_requires = >=3.6.1\n'
    )

$ pytest -q -k test_guess_python_envlist_py36 tests\setup_cfg_fmt_test.py
F                                                                                                                               [100%]
============================================================== FAILURES ==============================================================
___________________________________________________ test_guess_python_envlist_py36 ___________________________________________________

tmpdir = local('C:\\Users\\maxr\\AppData\\Local\\Temp\\pytest-of-maxr\\pytest-20\\test_guess_python_envlist_py360')

    def test_guess_python_envlist_py36(tmpdir):
        tmpdir.join('tox.ini').write('[tox]\nenvlist = py36\n')
        setup_cfg = tmpdir.join('setup.cfg')
        setup_cfg.write(
            '[metadata]\n'
            'name = pkg\n'
            'version = 1.0\n',
        )

        assert main((str(setup_cfg), ))

>       assert setup_cfg.read() == (
            '[metadata]\n'
            'name = pkg\n'
            'version = 1.0\n'
            'classifiers =\n'
            '    Programming Language :: Python :: 3\n'
            '    Programming Language :: Python :: 3 :: Only\n'
            '    Programming Language :: Python :: Implementation :: CPython\n'
            '\n'
            '[options]\n'
            'python_requires = >=3.6.1\n'
        )
E       AssertionError: assert '[metadata]\n...res = >=3.7\n' == '[metadata]\n...s = >=3.6.1\n'
E         Skipping 225 identical leading characters in diff, use -v to show
E         - res = >=3.6.1
E         ?           ^^^
E         + res = >=3.7
E         ?           ^

tests\setup_cfg_fmt_test.py:639: AssertionError
-------------------------------------------------------- Captured stdout call --------------------------------------------------------
Rewriting C:\Users\maxr\AppData\Local\Temp\pytest-of-maxr\pytest-20\test_guess_python_envlist_py360\setup.cfg
====================================================== short test summary info =======================================================
FAILED tests/setup_cfg_fmt_test.py::test_guess_python_envlist_py36 - AssertionError: assert '[metadata]\n...res = >=3.7\n' == '[metadata]\n...s = >=3.6.1\n'
1 failed, 66 deselected in 0.07s

yeah, setup-cfg-fmt takes tox as the source of truth -- that's the only way to make it stable (otherwise it would never update)

a bit unfortunate, but intentional

Hmm but here it's not taking tox as the source of truth when the version in tox is < the default min version of the tool. This is because the check here:

setup-cfg-fmt/setup_cfg_fmt.py

Lines 215 to 216 in aff5adb

    
           elif min_py3_version > minimum: 
        
               return _format_python_requires(min_py3_version, excluded)

can't discern if someone is specifying a min version explicitly or not.

It feels surprising for users of setup-cfg-fmt to have their own library's min version updated when min-py3-version's default changes (even if they still want to support an EOL'd Python version) - is that still intentional?

there's a trade off -- if it never decreases the value then the tox value would never get cleaned up

Got it thanks

	elif min_py3_version > minimum:
	return _format_python_requires(min_py3_version, excluded)