add 2-arg hash leveraging PyCall and PythonCall approaches

[jverzani]

This closes #520 leveraging code from both PyCall and PythonCall.

It also addresses https://discourse.julialang.org/t/sympy-sym-s-as-keys-in-dicts/103995

[codecov-commenter]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.10% 🎉

Comparison is base (9aa9a39) 55.92% compared to head (6a5482d) 56.02%.
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #522      +/-   ##
==========================================
+ Coverage   55.92%   56.02%   +0.10%     
==========================================
  Files          20       20              
  Lines        1250     1253       +3     
==========================================
+ Hits          699      702       +3     
  Misses        551      551              

Files Changed	Coverage Δ
src/assumptions.jl	18.40% <100.00%> (ø)
src/types.jl	38.46% <100.00%> (+3.76%)	⬆️

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.