PKG/DEP: Make importable without pytest

Closes pandas-dev#16065

Author: TomAugspurger

pandas/testing.py

@@ -4,5 +4,13 @@
 Public testing utility functions.
 """
 
-from pandas.util.testing import (
-    assert_frame_equal, assert_series_equal, assert_index_equal)
+try:
+    from pandas.util.testing import (
+        assert_frame_equal, assert_series_equal, assert_index_equal)
+except ImportError:
+    def _f(*args, **kwargs):
+        raise ImportError("pandas.testing requires pytest. Please "
+                          "`pip install pytest` or `conda install pytest`")
+    assert_frame_equal = _f
+    assert_series_equal_equal = _f
+    assert_index_equal = _f