Add reference to Django; import to the top

Author: troglotit

pandas/compat/__init__.py

@@ -36,6 +36,7 @@
 from unicodedata import east_asian_width
 import struct
 import inspect
+from collections import namedtuple
 
 PY2 = sys.version_info[0] == 2
 PY3 = (sys.version_info[0] >= 3)
@@ -70,8 +71,9 @@ def str_to_bytes(s, encoding=None):
     def bytes_to_str(b, encoding=None):
         return b.decode(encoding or 'utf-8')
 
+    # The signature version below is directly copied from Django,
+    # https://github.com/django/django/pull/4846
     def signature(f):
-        from collections import namedtuple
         sig = inspect.signature(f)
         args = [
             p.name for p in sig.parameters.values()