Stop using deprecated HTMLParser.unescape

HTMLParser.unescape is accessed even when unused - this will cause an
exception when `HTMLParser.unescape` is removed in Python 3.9.

changelog.d/1788.change.rst

+Changed compatibility fallback logic for ``html.unescape`` to avoid accessing ``HTMLParser.unescape`` when not necessary. ``HTMLParser.unescape`` is deprecated and will be removed in Python 3.9.

setuptools/py33compat.py

@@ -52,4 +52,8 @@ class Bytecode_compat:
 Bytecode = getattr(dis, 'Bytecode', Bytecode_compat)
 
 
-unescape = getattr(html, 'unescape', html_parser.HTMLParser().unescape)
+unescape = getattr(html, 'unescape', None)
+if unescape is None:
+    # HTMLParser.unescape is deprecated since Python 3.4, and will be removed
+    # from 3.9.
+    unescape = html_parser.HTMLParser().unescape