Add test for forward refs

[flying-sheep]

Fixes #14 once ready

I still can’t reproduce #14 locally, but I can here! Seems to have to do with dataclasses:

https://github.com/theislab/scanpydoc/runs/1165147987

cc @ded8393

[flying-sheep]

OK, progress I think. The generous help from pytest ❤️ tells us that:

1. The error happens for obj = <function __init__ at 0x7fa87f7db820>, options = {'members': []}

2. get_type_hints appears to be unable to find the module:

   globalns = localns = {
       'MISSING': <dataclasses._MISSING_TYPE object at 0x7fa880778a00>,
       '_HAS_DEFAULT_FACTORY': <factory>,
       '__builtins__': <module 'builtins' (built-in)>,
   }

I assume this is https://bugs.python.org/issue34776, and probably fixed in python/cpython#9518 which was backported to

• 3.7.6 ([3.7] bpo-34776: Fix dataclasses to support __future__ "annotations" mode (GH-9518) python/cpython#17532)
• 3.8.1 ([3.8] bpo-34776: Fix dataclasses to support __future__ "annotations" mode (GH-9518) python/cpython#17531)

---

Yo, WTF, they’re using ancient python patch versions 🤯:

• RTD (Update python versions readthedocs/readthedocs-docker-images#138)
• Travis

[codecov]

Codecov Report

Merging #20 into master will decrease coverage by 2.47%.
The diff coverage is 31.57%.

@@            Coverage Diff             @@
##           master      #20      +/-   ##
==========================================
- Coverage   85.67%   83.20%   -2.48%     
==========================================
  Files           9        9              
  Lines         363      381      +18     
==========================================
+ Hits          311      317       +6     
- Misses         52       64      +12     

Impacted Files	Coverage Δ
scanpydoc/elegant_typehints/return_tuple.py	76.38% <31.57%> (-14.36%)	⬇️

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