This updates recommonmark so that relative links to .md files will
correctly be converted to links to html pages by Sphinx/ReadTheDocs.
This was repoprted in https://github.com/rtfd/recommonmark/issues/89
This will allow us to in a future commit use relative links to .md
files so all of the hyperlinks work both on ReadTheDocs and on GitHub.
This commit renames various source requirements files like `dev.txt`,
`mypy.txt` etc to `dev.in`, `mypy.in` etc and various locked requirements
files like `dev_lock.txt`, `mypy_lock.txt` etc to `dev.txt`, `mypy.txt`
etc. This will help in emphasizing to the user that *.in are actually
input to `update-locked-requirements` tool which should be run after
updating any of these.