Use app.registry.documenters rather than get_documenters(app)

[domdfcoding]

get_documenters() has been deprecated since 3.0.0b1 but the changes to its internals occurred in sphinx-doc/sphinx#5370 , which made it into 2.0.0.

As the function just returns app.registry.documenters, and autodocsumm requires Sphinx>=2.2, there should be no problem switching away from the deprecated function.

[codecov]

Codecov Report

Merging #57 (17769dc) into master (3510543) will decrease coverage by 0.04%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #57      +/-   ##
==========================================
- Coverage   86.82%   86.77%   -0.05%     
==========================================
  Files           1        1              
  Lines         296      295       -1     
==========================================
- Hits          257      256       -1     
  Misses         39       39              

Impacted Files	Coverage Δ
autodocsumm/__init__.py	86.77% <100.00%> (-0.05%)	⬇️

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[Chilipp]

hey @domdfcoding! thanks a lot for your input =) looks good to me. let me know when this is ready to get merged

[domdfcoding]

I've tested this with a few of my projects just to make sure everything worked OK, and I think this is now ready for review.

[Chilipp]

awesome! thanks a lot @domdfcoding