Make use of Python 3.7's standard library importlib.resources

There is a hardcoded dependency on the third-party module
`importlib_resources`, which is `a backport of Python 3.7's
standard library importlib.resources module for Python 2.7,
and 3.4 through 3.6`. Thus, let's use the standard one for
Python3.7 and above.

Signed-off-by: Stanislav Levin <[email protected]>

Author: stanislavlevin

importlib_metadata/tests/test_zip.py

@@ -2,7 +2,10 @@
 import unittest
 import importlib_metadata
 
-from importlib_resources import path
+try:
+    from importlib_resources import path
+except ImportError:
+    from importlib.resources import path
 
 try:
     from contextlib import ExitStack

tox.ini

@@ -25,7 +25,7 @@ passenv =
 deps =
      cov,diffcov: coverage>=4.5
      diffcov: diff_cover
-     importlib_resources
+     importlib_resources; python_version < "3.7"
      py27: contextlib2
      pip >= 18
      packaging