Drop license classifier in favor of SPDX expression.

[leander-dsouza]

Description of contribution in a few bullet points

• Official Python packaging documentation now recommends using SPDX expressions instead of Trove's classifiers, and resolves the following warning:

  dist.py:759: SetuptoolsDeprecationWarning: License classifiers are deprecated.
  !!
  
          ********************************************************************************
          Please consider removing the following classifiers in favor of a SPDX license expression:
  
          License :: OSI Approved :: MIT License
  
          See https://packaging.python.org/en/latest/guides/writing-pyproject-toml/#license for details.
          ********************************************************************************
  !!
  

Description of how this change was tested

• No warnings when invoking python3 -m build.