Remove MANIFEST.in; Migrate to pyproject.toml

[bact]

Remove MANIFEST.in and migrate to pyproject.toml

• Consolidate config files for maintainability
• With the use of setuptools_scm, all files that are tracked with the version control system will be included in the distribution anyway -- so there's no need to declared "graft/include" in the MANIFEST.in.
• The side effect of completely remove MANIFEST.in is the egg-info will be included. As setuptools can't be configured to prune src/spdx_tools.egg-info via pyproject.toml config.
  • However, the inclusion of egg-info is actually a standard practice (setuptools itself also include this information).
  • The additional size of sdist when include the egg-info is 5.6 kbytes (704,357 vs 698,744 bytes).
  • If needed, we can also include a minimal MANIFEST.in with one line:

    prune src/spdx_tools.egg-info
    

    to exclude the egg-info from sdist

Modernize .gitignore

• Remove .Python, /pip-selfcheck.json, pip-log.txt -- from Python 2 era and obsoleted pip (that is no longer supported the minimum Python 3.10 requirement of current spdx-tools library)
• Group by category and sort alphabetically