`variant=CommonMark` gets stripped from `long_description_content_type`

[Jackenmen]

Minimal example:

• setup.cfg:

[metadata]
name = example_project
long_description = file: README.md
long_description_content_type = text/markdown; charset=UTF-8; variant=CommonMark

• README.md:

# Header

text

setup.cfg gets rewritten to:

[metadata]
name = example_project
long_description = file: README.md
long_description_content_type = text/markdown

This spec can be found here:
https://packaging.python.org/en/latest/specifications/core-metadata/#description-content-type

[asottile]

setup-cfg-fmt unconditionally assigns that value based on the detected content type of the readme file:

setup-cfg-fmt/setup_cfg_fmt.py

Lines 374 to 380 in d3e358c

	tags = identify.tags_from_filename(readme)
	if 'markdown' in tags:
	cfg['metadata']['long_description_content_type'] = 'text/markdown'
	elif 'rst' in tags:
	cfg['metadata']['long_description_content_type'] = 'text/x-rst'
	else:
	cfg['metadata']['long_description_content_type'] = 'text/plain'