Specify that the long description is rST in the package metadata. (#9180)

This avoids a warning when uploading packages to PyPI via twine.

changelog.d/9180.misc

+Add a `long_description_type` to the package metadata.

setup.py

@@ -121,6 +121,7 @@ setup(
     include_package_data=True,
     zip_safe=False,
     long_description=long_description,
+    long_description_content_type="text/x-rst",
     python_requires="~=3.5",
     classifiers=[
         "Development Status :: 5 - Production/Stable",