Fix Python 3.10+ issues from PY_SSIZE_T_CLEAN not being set

[chrisburr]

Writing files with XRootD and Python 3.10 currently fails with:

In [4]: with XRootD.client.File() as file:
   ...:     status, _ = file.open(f"{eos.url}{dest}", OpenFlags.NEW)
   ...:     assert status.ok, status
   ...:     status, _ = file.write(new_data)
   ...:     assert status.ok, status
   ...:
---------------------------------------------------------------------------
SystemError                               Traceback (most recent call last)
Input In [4], in <cell line: 1>()
      2 status, _ = file.open(f"{eos.url}{dest}", OpenFlags.NEW)
      3 assert status.ok, status
----> 4 status, _ = file.write(new_data)
      5 assert status.ok, status

File ~/mambaforge/envs/logse-sqlite/lib/python3.10/site-packages/XRootD/client/file.py:190, in File.write(self, buffer, offset, size, timeout, callback)
    187   callback = CallbackWrapper(callback, None)
    188   return XRootDStatus(self.__file.write(buffer, offset, size, timeout, callback))
--> 190 status, response = self.__file.write(buffer, offset, size, timeout)
    191 return XRootDStatus(status), None

SystemError: PY_SSIZE_T_CLEAN macro must be defined for '#' formats

This PR fixes the issue by defining PY_SSIZE_T_CLEAN as suggested. Though I'm leaving it as a draft at the moment as I still need to change int to Py_ssize_t in the various places to be fully correct.

There is no issue with backwards compatibilty as support for PY_SSIZE_T_CLEAN was added in Python 2.5

See the release notes and PEP-353 for details.

[simonmichal]

@chrisburr : thanks a lot for the PR, just let me know once it's ready for review!

[chrisburr]

I think this is ready.

The test failure seems to be unrelated but if it's not expected to fail I'll look into it further.