Add blackdoc to format Python codes in docstrings

[seisman]

Description of proposed changes

This PR adds blackdoc to format doctests and also wrap doctests to 79 characters, as proposed in #497 and #384.

There are two commits in this PR. It's easier to review each commit separately.

• 3d86357: Add blackdoc support
• a92fa5b: Run blackdoc to format docstrings

Address #497.

Reminders

• Run make format and make check to make sure the code follows the style guide.
• Add tests for new features or tests that would have caught the bug that you're fixing.
• Add new public functions/methods/classes to doc/api/index.rst.
• Write detailed docstrings for all functions/methods.
• If adding new functionality, add an example to docstrings or tutorials.

[weiji14]

Looks nice! Still need to seek out a tool to wrap the docstrings themselves to 79 characters, but one step at a time 😄