fix spelling of 'readme.md' -> 'readme.MD' in pyproject.toml

[hemidactylus]

Otherwise a fresh poetry install would error by mismatch in md != MD:

$ poetry install
Installing dependencies from lock file

Package operations: 49 installs, 1 update, 0 removals

  • Installing click (8.1.7)
...
...
  • Installing types-toml (0.10.8.7)

[Errno 2] No such file or directory: '/home/stefano/personal/Datastax/code/other_clones/astrapy/README.md'

[cbornet]

If you do that, poetry build doesn't use the readme for the package description.

[hemidactylus]

Not the right solution (If you do that, poetry build doesn't use the readme for the package description)