Replace pkg_resources with packaging in setup.py

setuptools>=78 removed the bundled pkg_resources module, causing
ModuleNotFoundError on all CI jobs except Python 3.8-ubuntu (which
uses an older setuptools). Use packaging.requirements.Requirement
instead, which provides the same version-specifier matching and is
always available as a dependency of setuptools.

Author: veeceey

setup.py

@@ -108,7 +108,7 @@ def finalize_options(self):
                         need_cythonize = True
 
         if need_cythonize:
-            import pkg_resources
+            from packaging.requirements import Requirement
 
             # Double check Cython presence in case setup_requires
             # didn't go into effect (most likely because someone
@@ -121,8 +121,8 @@ def finalize_options(self):
                     'please install {} to compile uvloop from source'.format(
                         CYTHON_DEPENDENCY))
 
-            cython_dep = pkg_resources.Requirement.parse(CYTHON_DEPENDENCY)
-            if Cython.__version__ not in cython_dep:
+            cython_req = Requirement(CYTHON_DEPENDENCY)
+            if not cython_req.specifier.contains(Cython.__version__):
                 raise RuntimeError(
                     'uvloop requires {}, got Cython=={}'.format(
                         CYTHON_DEPENDENCY, Cython.__version__