setup.cfg: Replace dashes by underscores

Since setuptools v54.1.0[1], the parmeters with dash have been
deprecated in favor of the new parameters with underscore.

This change updates the parameters accordingly to avoid the warnings
like the example below.

  UserWarning: Usage of dash-separated 'description-file' will not be
  supported in future versions. Please use the underscore name
  'description_file' instead

[1] https://github.com/pypa/setuptools/commit/a2e9ae4cb

Change-Id: I2d1e9c87c87071716aada93fc4c1e93ed3880ca9

setup.cfg

@@ -1,9 +1,9 @@
 [metadata]
 name = python-mistralclient
 summary = Mistral Client Library
-description-file = README.rst
+description_file = README.rst
 license = Apache Software License
-python-requires = >=3.9
+python_requires = >=3.9
 classifiers =
     Programming Language :: Python
     Programming Language :: Python :: Implementation :: CPython
@@ -18,8 +18,8 @@ classifiers =
     License :: OSI Approved :: Apache Software License
     Operating System :: POSIX :: Linux
 author = OpenStack
-author-email = openstack-discuss@lists.openstack.org
-home-page = https://docs.openstack.org/python-mistralclient/latest/
+author_email = openstack-discuss@lists.openstack.org
+home_page = https://docs.openstack.org/python-mistralclient/latest/
 
 [files]
 packages =