Do not import pandas and related modules if it is not patched

- see #627

Author: mrbean-bremen

CHANGES.md

@@ -8,6 +8,8 @@ The released versions correspond to PyPi releases.
 * improved handling of `str`/`bytes` paths
 * suppress all warnings while inspecting loaded modules
   (see [#614](../../issues/614))
+* do not import pandas and related modules if it is not patched
+  (see [#627](../../issues/627))
 
 ### Infrastructure
 * added test dependency check (see [#608](../../issues/608))

pyfakefs/fake_filesystem_unittest.py

@@ -59,9 +59,6 @@
 )
 from pyfakefs.helpers import IS_PYPY
 from pyfakefs.mox3_stubout import StubOutForTesting
-from pyfakefs.patched_packages import (
-    get_modules_to_patch, get_classes_to_patch, get_fake_module_classes
-)
 
 try:
     from importlib.machinery import ModuleSpec
@@ -474,6 +471,11 @@ def __init__(self, additional_skip_names: Optional[
         self.use_cache = use_cache
 
         if use_known_patches:
+            from pyfakefs.patched_packages import (
+                get_modules_to_patch, get_classes_to_patch,
+                get_fake_module_classes
+            )
+
             modules_to_patch = modules_to_patch or {}
             modules_to_patch.update(get_modules_to_patch())
             self._class_modules.update(get_classes_to_patch())