[fix] TypeError coming from suffix arg removed from sphinx v3.0.0

[CrossNox]

Since 1.8.0, the suffix argument for app.add_source_parser() has been deprecated. And it is removed now.

Fix and mantain backwards compatibility for older sphinx versions.

Closes #51

[matthewfeickert]

@miyakogi If you get time to review this it would be great.

[basnijholt]

@miyakogi ping. No project that uses m2r builds using the latest Sphinx. This includes all projects on Readthedocs.io (that use m2r).

[sbliven]

@basnijholt You can try @qhua948's fork, published as m2rr.

[CrossNox]

I've finally released my fork as m2r2 after revamping some of the tooling and building CD and testing pipelines.

[basnijholt]

Any comments on how https://github.com/qhua948/m2rr and https://github.com/CrossNox/m2r2 differ? Maybe only time will tell us which one will be maintained 😄

[axsaucedo]

This is great, thanks guys, will make a massive difference once it's merged. I'm now using m2r2 and seems to work quite well 👍

[codecov-io]

Codecov Report

Merging #55 (ee7c818) into dev (66f4a5a) will increase coverage by 0.13%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##              dev      #55      +/-   ##
==========================================
+ Coverage   96.14%   96.27%   +0.13%     
==========================================
  Files           3        3              
  Lines         778      779       +1     
==========================================
+ Hits          748      750       +2     
+ Misses         30       29       -1     

Impacted Files	Coverage Δ
m2r.py	93.59% <100.00%> (+0.32%)	:arrow_up:

---

Continue to review full report at Codecov.

Legend - Click here to learn more Δ = absolute <relative> (impact), ø = not affected, ? = missing data Powered by Codecov. Last update 66f4a5a...ee7c818. Read the comment docs.