twine check should validate Description-Content-Type

[twm]

Your Environment

1. Your operating system: Ubuntu 18.04

2. Version of python you are running: Python 3.7.7 (distro version)

3. How did you install twine? Did you use your operating system's package manager or pip or something else?

pipx (so pip)

4. Version of twine you have installed (include complete output of):

twine version 3.1.1 (pkginfo: 1.5.0.1, requests: 2.23.0, setuptools: 46.0.0,
requests-toolbelt: 0.9.1, tqdm: 4.43.0)

5. Which package repository are you targeting? pypi.org

The Issue

twine check doesn't validate Description-Content-Type / long_description_content_type.

Steps to Reproduce

I added Description-Content-Type metadata because twine check warned me it was missing, but I made a typo: it is test/x-rst, (test vs. text).

I ran twine check on the packages with this metadata:

twine check dist/*.*
Checking dist/treq-20.3.0.tar.gz: PASSED
Checking dist/treq-20.4.0.tar.gz: PASSED

However, when I went to upload to PyPI:

twine upload dist/treq-20.4.0*
Uploading distributions to https://upload.pypi.org/legacy/
Enter your username: twm
Enter your password: 
Uploading treq-20.4.0-py2.py3-none-any.whl
100%|████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████| 62.2k/62.2k [00:01<00:00, 44.6kB/s]
NOTE: Try --verbose to see response content.
HTTPError: 400 Client Error: 'test/x-rst' is an invalid value for Description-Content-Type. Error: Invalid description content type: type/subtype is not valid See https://packaging.python.org/specifications/core-metadata for url: https://upload.pypi.org/legacy/

The artifacts are attached here: https://github.com/twisted/treq/releases/tag/release-20.4.0

[di]

Ultimately I think this is a duplicate of #430 and will be resolved when we begin validating all the metadata.

[bhrutledge]

@di I've labeled this is blocked, unless you think it's preferable to close it as a duplicate.

[bhrutledge]

After doing some triage, going to close this as a duplicate of #430.