Fix flake8-bugbear warning

B009 Do not call `getattr` with a constant attribute value.
     It is not any safer than normal property access.

Fix a MyPy typing issue, instead of using a Python hack to
silence the PyPy error:
	error: Incompatible types in assignment (expression
	       has type "Optional[str]", variable has type
	       "str")  [assignment]

src/packaging/_manylinux.py

@@ -82,7 +82,7 @@ def _glibc_version_string_confstr() -> Optional[str]:
     # https://github.com/python/cpython/blob/fcf1d003bf4f0100c/Lib/platform.py#L175-L183
     try:
         # Should be a string like "glibc 2.17".
-        version_string: str = getattr(os, "confstr")("CS_GNU_LIBC_VERSION")
+        version_string: Optional[str] = os.confstr("CS_GNU_LIBC_VERSION")
         assert version_string is not None
         _, version = version_string.rsplit()
     except (AssertionError, AttributeError, OSError, ValueError):