chore: Add noxfile to run pytest across multiple python versions

[Higgcz]

This PR adds a configuration script for Nox.

Nox is useful mainly when you finish working on some bigger feature and you want to be sure it works in all supported versions of Python.

In order to use Nox, just install it:

pip install nox

And run it:

nox

Nox will create 4 independent environments - this takes quite a while, so I've set Nox to reuse already built environments, which means if you change something inside an environment, you need to manually remove it from .nox/ directory.

As the next step, we should run all Python versions tests inside Github Actions.

[codecov-commenter]

Codecov Report

Merging #40 (b1ae33a) into master (d3e3f0a) will decrease coverage by 0.78%.
The diff coverage is n/a.

❗ Current head b1ae33a differs from pull request most recent head eba10ac. Consider uploading reports for the commit eba10ac to get more accurate results

@@            Coverage Diff             @@
##           master      #40      +/-   ##
==========================================
- Coverage   52.97%   52.19%   -0.79%     
==========================================
  Files          43       43              
  Lines        6233     6246      +13     
==========================================
- Hits         3302     3260      -42     
- Misses       2931     2986      +55     

Impacted Files	Coverage Δ
src/evotorch/core.py	61.59% <0.00%> (-3.96%)	⬇️
src/evotorch/operators/base.py	33.01% <0.00%> (+0.30%)	⬆️

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[NaturalGradient]

I have tried it out and barring the addition of the swig dependency, which is now added, it works perfectly!