fix 20ms startup penalty added by virtualenv (#2317)

docs/changelog/2317.bugfix.rst

@@ -0,0 +1 @@
+Improve performance of python startup inside created virtualenvs - by :user:`asottile`.

docs/changelog/examples.rst

@@ -2,17 +2,17 @@
 
 file ``544.doc.rst``::
 
-    explain everything much better - by ``passionate_technicalwriter``
+    explain everything much better - by :user:`passionate_technicalwriter`.
 
 file ``544.feature.rst``::
 
-    ``tox --version`` now shows information about all registered plugins - by ``obestwalter``
+    ``tox --version`` now shows information about all registered plugins - by :user:`obestwalter`.
 
 
 file ``571.bugfix.rst``::
 
     ``skip_install`` overrides ``usedevelop`` (``usedevelop`` is an option to choose the
     installation type if the package is installed and ``skip_install`` determines if it should be
-    installed at all) - by ``ferdonline``
+    installed at all) - by :user:`ferdonline`.
 
 .. see pyproject.toml for all available categories

src/virtualenv/create/via_global_ref/_virtualenv.py

@@ -37,11 +37,8 @@ def parse_config_files(self, *args, **kwargs):
 _DISTUTILS_PATCH = "distutils.dist", "setuptools.dist"
 if sys.version_info > (3, 4):
     # https://docs.python.org/3/library/importlib.html#setting-up-an-importer
-    from functools import partial
-    from importlib.abc import MetaPathFinder
-    from importlib.util import find_spec
 
-    class _Finder(MetaPathFinder):
+    class _Finder:
         """A meta path finder that allows patching the imported distutils modules"""
 
         fullname = None
@@ -65,6 +62,9 @@ def find_spec(self, fullname, path, target=None):
                     # https://docs.python.org/3/faq/library.html#what-kinds-of-global-value-mutation-are-thread-safe
                     self.lock.append(lock)
 
+                from functools import partial
+                from importlib.util import find_spec
+
                 with self.lock[0]:
                     self.fullname = fullname
                     try: