Misc fixes

[skirpichev]

• update default version (to run coverage tests and for docs)
• fix hang in polylog_general(), closes Hang in polylog function with certain special value inputs #505
• fix demos, closes some demos broken #216
• change warning category for force_type to DeprecationWarning, closes fix force_type for matrices #156
• run no-gmpy2 tests on pypy
• run tests on CPython 3.13 (no gmpy2)
• use asserts in normalize(), drop MPMATH_STRICT
• use cholesky_solve() for overdetermined complex linear systems, closes overdetermined complex linear equation systems #227
• update versions for GA actions
• fix several issues with empty matrices, close Null matrices don't properly convert to repr string and attempting to compute the determinant of 0×0 matrix raised an exception. #745
• cleanup test.yml, run no-gmpy tests on 3.11
• fix few more py2 remnants (__truediv__ vs __div__)
• drop globals row/colsep from the matrices module
• don't use private name mangling in the matrices.py
• depend on the matplotlib (for testing), filter a warning
• relax numpy reqs
• update _matrix.rows/cols
• revert update for codecov-action (v4 requires a token)