test_pluginmanager: use pluggy.manager.metadata

[blueyed]

This has the version switch in a central place only once.

Ref: https://github.com/pytest-dev/pluggy/pull/223/files#r315505111

[bluetech]

manager.metadata doesn't read too well to me, it's not immediately clear that it refers to importlib.metadata.

It will be clearer if, in manager.py, the common name is changed to importlib_metadata, that is

 if sys.version_info >= (3, 8):
-    from importlib import metadata
+    from importlib import metadata as importlib_metadata
 else:
-     import importlib_metadata as metadata
+     import importlib_metadata

[bluetech]

(And also, do from pluggy.manager import importlib_metadata instead of using it qualified).

[blueyed]

Done.