Use m2r2 instead of m2r

[cairosanders]

This pull request resolves #130

It looks like m2r is no longer maintained but I found m2r2 which is forked off the original m2r repository and it seems to work with sphinx version > 3 although I'm not 100% confident on what the expected output is when you run python setup.py build_sphinx...

[corviday]

I built the documentation in both HTML and PDF, which was a smooth process with a correct result. Looks good!

There were two error messages during the build that were not introduced by these changes, but seem to have been revealed by newly improved error checking. They seem pretty minor, would it be possible to correct them as part of this PR? If not, not problem, I'll just make a new issue.

• mismatched single quotes around attribute=value on line 4 of doc/source/api/api-overview.md; the quotes should both be backticks. Results in a nonsensical link.
• the paragraph that begins "When requesting streamflow data from a point" in doc/source/workflow.md has spaces at the beginning of the last few lines of the paragraph. Results in a weird indented section for no discernable reason.

[cairosanders]

I can make those changes first thing tomorrow and add them to this pull request

[corviday]

Whoops, found one more bit of screwy help formatting, also in doc/source/workflow.md. A line begins with "1990. The February timestamp" which is interpreted by m2r2 as a numerical list, like this:

Moving the 1990 to the previous line would fix this, I think.

Sorry for dumping a bunch of unrelated changes on you, thanks for making the helpfile building so much easier!

[cairosanders]

I think that should solve the problems

[corviday]

Looks great.