📦 Stop marking arbitrary files as package data

Previously, any files included in sdist were marked as installable
data which confused `setuptools` and made it issue a warning.

This patch unsets the setting to prevent that.

Fixes #829.

setup.py

@@ -83,8 +83,6 @@ def read(f):
     license="Apache 2",
     packages=["multidict"],
     python_requires=">=3.7",
-    include_package_data=True,
-    exclude_package_data={"": ["*.c", "*.h"]},
 )
 
 if not NO_EXTENSIONS: