py-commonmark: new port

[harens]

Description

In the hopes of adding rich to MacPorts (which is now possible thanks to python-poetry/poetry#1975), I wrote a portfile for one of its dependencies.

Type(s)

• bugfix
• enhancement
• security fix

Tested on

macOS 10.15.6 19G73
Xcode 12.0 12A7209

Verification

Have you

• followed our Commit Message Guidelines?
• squashed and minimized your commits?
• checked that there aren't other open pull requests for the same change?
• referenced existing tickets on Trac with full URL?
• checked your Portfile with port lint?
• tried existing tests with sudo port test?
• tried a full install with sudo port -vst install?
• tested basic functionality of all binary files?

[macportsbot]

Travis Build #14402 Passed.

Lint results

--->  Verifying Portfile for py-commonmark
--->  0 errors and 0 warnings found.

Port py38-commonmark success on xcode10.3. Log
Port py-commonmark success on xcode10.3. Log
Port py38-commonmark success on xcode9.4. Log
Port py-commonmark success on xcode9.4. Log
Port py38-commonmark success on xcode8.3. Log
Port py-commonmark success on xcode8.3. Log
Port py38-commonmark success on xcode7.3. Log
Port py-commonmark success on xcode7.3. Log