MAINT/BLD: Remove Cython as a runtime requirement

[bocklund]

Cython is not a runtime requirement and can be removed. It should stay in pyproject.toml and requirements-dev.txt since it is required at build time.

Hopefully requirements-dev.txt can be removed after pypa/setuptools#2816 is complete and pip install -e . is PEP 517-compatible (i.e. PEP 660).

Checklist

• If any dependencies have changed, the changes are reflected in the
  • setup.py (runtime requirements)
  • pyproject.toml (build requirements)
  • requirements-dev.txt (build and development requirements)

[codecov]

Codecov Report

Merging #380 (ccb5230) into develop (77f25eb) will decrease coverage by 0.00%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##           develop     #380      +/-   ##
===========================================
- Coverage    89.93%   89.92%   -0.01%     
===========================================
  Files           44       44              
  Lines         4370     4368       -2     
===========================================
- Hits          3930     3928       -2     
  Misses         440      440              

Impacted Files	Coverage Δ
pycalphad/plot/eqplot.py	68.88% <0.00%> (-0.68%)	⬇️

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