🔧 Update CodeQL configuration

[burgholzer]

This PR updates the GitHub CodeQL configuration in anticipation of the deprecation of LGTM.
Most importantly, it adds further checks and a custom build step for C++ to also cover the Python bindings.

These additional checks have revealed a couple of easy to fix warnings that have subsequently been fixed.
As a pleasant side effect, the Python tests now work irrespective of the directory they are executed in.
A corresponding nox session tests (similar to lint, and docs) is added to the project.

Conveniently run the Python tests via

nox -rs tests

Use nox -rs tests-3.9 to run the tests on Python 3.9 only.

[codecov]

Codecov Report

Merging #75 (973da7b) into main (58ee277) will not change coverage.
The diff coverage is 0.0%.

@@          Coverage Diff          @@
##            main     #75   +/-   ##
=====================================
  Coverage   91.2%   91.2%           
=====================================
  Files         29      29           
  Lines       1825    1825           
=====================================
  Hits        1666    1666           
  Misses       159     159           

Impacted Files	Coverage Δ
include/core/syrec/grammar.hpp	95.8% <ø> (ø)
include/core/syrec/module.hpp	90.9% <ø> (ø)
include/core/syrec/statement.hpp	95.3% <ø> (ø)
src/core/syrec/parser.cpp	86.4% <0.0%> (ø)

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