fix compatiblity with Python 3.7

pdoc/render.py

@@ -3,9 +3,8 @@
 import os
 import types
 import warnings
-from collections.abc import Mapping
 from pathlib import Path
-from typing import cast
+from typing import cast, Mapping
 
 import jinja2
 from jinja2 import Environment, FileSystemLoader

pdoc/web.py

@@ -12,7 +12,8 @@
 import traceback
 import warnings
 import webbrowser
-from collections.abc import Iterable, Iterator, Mapping
+from collections.abc import Iterable, Iterator
+from typing import Mapping
 
 from pdoc import doc, extract, render
 from pdoc._compat import cache, removesuffix