README isn't rendered correctly on PyPI

[Changaco]

Link.

(Reticketed from #2 (comment).)

[Changaco]

I've found the command to debug this issue and prevent it from happening again: twine check dist/*.

It currently gives the following error message:

The project's long_description has invalid markup which will not be rendered on PyPI. The following syntax errors were detected:
line 1: Error: Unknown target name: "aspen.io".