Wrap cached_property import in try/except for Python 3.7.

Author: rossbar

numpydoc/docscrape.py

@@ -8,11 +8,17 @@
 from warnings import warn
 from collections import namedtuple
 from collections.abc import Callable, Mapping
-from functools import cached_property
 import copy
 import sys
 
 
+# TODO: Remove try-except when support for Python 3.7 is dropped
+try:
+    from functools import cached_property
+except ImportError:  # cached_property added in Python 3.8
+    cached_property = None
+
+
 def strip_blank_lines(l):
     "Remove leading and trailing blank lines from a list of lines"
     while l and not l[0].strip():