DOC: Separate out readthedocs requirements

[shaneahmed]

• Separate out readthedocs requirements to avoid python 3.7 requirements errors.

[codecov]

Codecov Report

Merging #227 (e3240cd) into develop (49a2baf) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff            @@
##           develop     #227   +/-   ##
========================================
  Coverage    99.81%   99.81%           
========================================
  Files           53       53           
  Lines         4867     4867           
  Branches       800      800           
========================================
  Hits          4858     4858           
  Misses           2        2           
  Partials         7        7           

Impacted Files	Coverage Δ
tiatoolbox/models/architecture/hovernet.py	100.00% <ø> (ø)
tiatoolbox/wsicore/wsireader.py	99.12% <ø> (ø)

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