Remove dependency of pyfakefs on pytest

- see pytest-dev#565

CHANGES.md

@@ -3,6 +3,10 @@ The released versions correspond to PyPi releases.
 
 ## Version 4.3.0 (as yet unreleased)
 
+### Fixes
+  * remove dependency of pyfakefs on `pytest` (regression, 
+    see [#565](../../issues/565)) 
+
 ## [Version 4.2.0](https://pypi.python.org/pypi/pyfakefs/4.2.0)
 
 #### New Features

pyfakefs/fake_filesystem_unittest.py

@@ -46,9 +46,6 @@
 import unittest
 import warnings
 
-import py
-import pytest
-
 from pyfakefs.deprecator import Deprecator
 from pyfakefs.fake_filesystem import set_uid, set_gid, reset_ids, PatchMode
 from pyfakefs.helpers import IS_PYPY
@@ -354,8 +351,16 @@ class Patcher:
     '''
     SKIPMODULES = {
         None, fake_filesystem, fake_filesystem_shutil,
-        sys, linecache, tokenize, py, pytest
+        sys, linecache, tokenize
     }
+    try:
+        import py
+        import pytest
+        SKIPMODULES += py
+        SKIPMODULES += pytest
+    except ImportError:
+        pass
+
     assert None in SKIPMODULES, ("sys.modules contains 'None' values;"
                                  " must skip them.")