deprecation warning on regex

[Frank Bergmann]

Running the tests, we see the following deprecation warnings:

=============================== warnings summary ===============================
pyenzyme/enzymeml/tools/unitparser.py:30
  /home/runner/work/PyEnzyme/PyEnzyme/pyenzyme/enzymeml/tools/unitparser.py:30: DeprecationWarning: invalid escape sequence \d
    regex = "([a-zA-Z]*)([-+][\d]*)"

pyenzyme/enzymeml/tools/unitparser.py:51
  /home/runner/work/PyEnzyme/PyEnzyme/pyenzyme/enzymeml/tools/unitparser.py:51: DeprecationWarning: invalid escape sequence \d
    regex = "([a-zA-z]*)([-+]?\d*)"

-- Docs: https://docs.pytest.org/en/stable/warnings.html
======================== 4 passed, 2 warnings in 1.01s =========================

[Jan Range]

The warnings were resolved by converting the strings to raw strings. Thanks for pointing out!