Fix mpmath deprecation warnings

[kwabenantim]

Summary

These warnings are present from Python 3.9+

=============================== warnings summary ===============================
../../../../../opt/hostedtoolcache/Python/3.14.5/x64/lib/python3.14/site-packages/mpmath/ctx_mp.py:35
  /opt/hostedtoolcache/Python/3.14.5/x64/lib/python3.14/site-packages/mpmath/ctx_mp.py:35: DeprecationWarning: mpnumeric is deprecated
    warnings.warn(f"{name} is deprecated", DeprecationWarning)

../../../../../opt/hostedtoolcache/Python/3.14.5/x64/lib/python3.14/site-packages/mpmath/rational.py:4
../../../../../opt/hostedtoolcache/Python/3.14.5/x64/lib/python3.14/site-packages/mpmath/rational.py:4
../../../../../opt/hostedtoolcache/Python/3.14.5/x64/lib/python3.14/site-packages/mpmath/rational.py:4
  /opt/hostedtoolcache/Python/3.14.5/x64/lib/python3.14/site-packages/mpmath/rational.py:4: DeprecationWarning: the rational private module is deprecated
    warnings.warn("the rational private module is deprecated",

chaste_codegen/tests/test_codegen.py: 175 warnings
chaste_codegen/tests/test_console_script.py: 89 warnings
chaste_codegen/tests/test_jacobian.py: 7 warnings
chaste_codegen/tests/test_linearity_check.py: 1 warning
chaste_codegen/tests/test_printer.py: 5 warnings
  /opt/hostedtoolcache/Python/3.14.5/x64/lib/python3.14/site-packages/mpmath/libmp/libintmath.py:75: DeprecationWarning: bitcount function is deprecated
    warnings.warn("bitcount function is deprecated",

-- Docs: https://docs.pytest.org/en/stable/how-to/capture-warnings.html