Add a readthedocs configuration file

[rtpg]

This is an attempt to get the readthedocs.io builds working again

[codecov[bot]]

Codecov Report

Merging #879 (e2cffd8) into master (1b241a3) will decrease coverage by 0.85%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #879      +/-   ##
==========================================
- Coverage   92.31%   91.47%   -0.85%     
==========================================
  Files           9        9              
  Lines         729      727       -2     
  Branches      131      112      -19     
==========================================
- Hits          673      665       -8     
- Misses         36       41       +5     
- Partials       20       21       +1     

Files	Coverage Δ
taggit/__init__.py	100.00% <100.00%> (ø)

... and 3 files with indirect coverage changes

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