fix: Use build_requires in setup.cfg

[matthewfeickert]

In the sdist creation phase of the build there is currently the warning:

$ python setup.py sdist
/home/runner/work/panda-client/panda-client/setup.py:6: DeprecationWarning: The distutils package is deprecated and slated for removal in Python 3.12. Use setuptools or check PEP 632 for potential alternatives
  import distutils
/opt/hostedtoolcache/Python/3.10.0/x64/lib/python3.10/site-packages/setuptools/dist.py:697: UserWarning: Usage of dash-separated 'build-requires' will not be supported in future versions. Please use the underscore name 'build_requires' instead
  warnings.warn(
...

panda-client/setup.cfg

Lines 3 to 7 in 3c2aee0

	[bdist_rpm]
	provides = panda-client
	release = 1
	packager = Panda Team <hn-atlas-panda-pathena@cern.ch>
	build-requires = python-devel

This PR addresses the setuptools warning RE: using build_requires in setup.cfg.

The distuitls warning should be addressed in a separate PR.

[matthewfeickert]

@tmaeno this is ready for review. Please let me know if you have any questions.