Use importlib.metadata from the standard library on Python 3.8+

Fixes tox-dev#1367

CONTRIBUTORS

@@ -58,6 +58,7 @@ Mattieu Agopian
 Michael Manganiello
 Mickaël Schoentgen
 Mikhail Kyshtymov
+Miro Hrončok
 Monty Taylor
 Morgan Fainberg
 Nick Douma

docs/changelog/1367.misc.rst

@@ -0,0 +1,2 @@
+Replace ``importlib_metadata`` backport with ``importlib.metadata``
+from the standard library on Python ``3.8+`` - by :user:`hroncok`

setup.cfg

@@ -38,7 +38,7 @@ classifiers =
 packages = find:
 python_requires = >=2.7, !=3.0.*, !=3.1.*, !=3.2.*, !=3.3.*
 install_requires =
-    importlib-metadata >= 0.12, <1
+    importlib-metadata >= 0.12, <1;python_version<"3.8"
     packaging >= 14
     pluggy >= 0.12.0, <1
     py >= 1.4.17, <2

src/tox/config/__init__.py

@@ -15,7 +15,6 @@
 from subprocess import list2cmdline
 from threading import Thread
 
-import importlib_metadata
 import pluggy
 import py
 import toml
@@ -34,6 +33,7 @@
     verbosity1,
 )
 from tox.util.path import ensure_empty_dir
+from tox.util.stdlib import importlib_metadata
 
 from .parallel import ENV_VAR_KEY as PARALLEL_ENV_VAR_KEY
 from .parallel import add_parallel_config, add_parallel_flags

src/tox/session/commands/show_config.py

@@ -1,13 +1,13 @@
 import sys
 from collections import OrderedDict
 
-import importlib_metadata
 from packaging.requirements import Requirement
 from packaging.utils import canonicalize_name
 from six import StringIO
 from six.moves import configparser
 
 from tox import reporter
+from tox.util.stdlib import importlib_metadata
 
 DO_NOT_SHOW_CONFIG_ATTRIBUTES = (
     "interpreters",

src/tox/util/stdlib.py

@@ -3,6 +3,11 @@
 from contextlib import contextmanager
 from tempfile import TemporaryFile
 
+if sys.version_info >= (3, 8):
+    from importlib import metadata as importlib_metadata  # noqa
+else:
+    import importlib_metadata  # noqa
+
 
 def is_main_thread():
     """returns true if we are within the main thread"""

tox.ini

@@ -139,7 +139,7 @@ include_trailing_comma = True
 force_grid_wrap = 0
 line_length = 99
 known_first_party = tox,tests
-known_third_party = apiclient,docutils,filelock,flaky,freezegun,git,httplib2,importlib_metadata,oauth2client,packaging,pathlib2,pluggy,py,pytest,setuptools,six,sphinx,toml
+known_third_party = apiclient,docutils,filelock,flaky,freezegun,git,httplib2,oauth2client,packaging,pathlib2,pluggy,py,pytest,setuptools,six,sphinx,toml
 
 [testenv:release]
 description = do a release, required posarg of the version number