Fix sympy compat and enhance formula eval

[J08nY]

Fixes #65 and improves performance of formula evaluation and Mod's siginificantly.

[codecov]

Codecov Report

Attention: Patch coverage is 82.74510% with 44 lines in your changes missing coverage. Please review.

Project coverage is 87.21%. Comparing base (204f3ed) to head (b61e1f7).

Files	Patch %	Lines
pyecsca/ec/mod.py	82.98%	27 Missing and 5 partials ⚠️
pyecsca/ec/formula/base.py	85.19%	4 Missing and 4 partials ⚠️
pyecsca/ec/curve.py	50.00%	0 Missing and 2 partials ⚠️
pyecsca/misc/cfg.py	0.00%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #66      +/-   ##
==========================================
- Coverage   87.51%   87.21%   -0.30%     
==========================================
  Files          86       86              
  Lines        7549     7707     +158     
  Branches     1422     1444      +22     
==========================================
+ Hits         6606     6721     +115     
- Misses        660      697      +37     
- Partials      283      289       +6     

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