Replace importlib_metadata with importlib.metadata on Python 3.8+

Fixes https://github.com/pytest-dev/pytest/issues/5537

src/_pytest/compat.py

@@ -26,6 +26,12 @@ MODULE_NOT_FOUND_ERROR = (
 )
 
 
+if sys.version_info >= (3, 8):
+    from importlib import metadata as importlib_metadata  # noqa
+else:
+    import importlib_metadata  # noqa
+
+
 def _format_args(func):
     return str(signature(func))
 

src/_pytest/config/__init__.py

@@ -9,7 +9,6 @@ import types
 import warnings
 from functools import lru_cache
 
-import importlib_metadata
 import py
 from packaging.version import Version
 from pluggy import HookimplMarker
@@ -25,6 +24,7 @@ from .findpaths import determine_setup
 from .findpaths import exists
 from _pytest._code import ExceptionInfo
 from _pytest._code import filter_traceback
+from _pytest.compat import importlib_metadata
 from _pytest.outcomes import fail
 from _pytest.outcomes import Skipped
 from _pytest.warning_types import PytestConfigWarning