"twine check" is emitting a UserWarning when executed because it's misusing readme_renderer

[mmerickel]

1. Your operating system:

macOS 10.14

2. Version of python you are running:

3.7.0

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

pip

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

1.12.1

The Issue

Please describe the issue that you are experiencing.

Twine is unconditionally importing readme_renderer.markdown but is not depending on readme_renderer[md], thus triggering a UserWarning when twine check is executed.

Steps to Reproduce

run twine check and observe

/path/to/virtualenv/lib/python3.6/site-packages/readme_renderer/markdown.py:38: UserWarning: Markdown renderers are not available. Install 'readme_render[md]' to enable Markdown rendering.
  warnings.warn(_EXTRA_WARNING)

Proposed fix

Do not import readme_renderer.markdown unless the content-type text/markdown is found. Then when validation fails the user sees the warning and knows they need to install the extra dependency.

twine/twine/commands/check.py

Line 27 in bd1d8b0

import readme_renderer.markdown

[di]

I'm planning to fix this.

[nok]

Nice, when will it be released (1.12.3)?