Pin to `pdbpp` upstream master, 3.10 problem?

See issues:
- https://github.com/pdbpp/pdbpp/issues/480
- https://github.com/pdbpp/pdbpp/pull/482

setup.py

@@ -54,7 +54,10 @@ setup(
         # tooling
         'colorlog',
         'wrapt',
-        'pdbpp',
+
+        # 3.10 has an outstanding unreleased issue and `pdbpp` itself
+        # pins to patched forks of its own dependencies as well.
+        "pdbpp @ git+https://github.com/pdbpp/pdbpp@master#egg=pdbpp",  # noqa: E501
 
         # serialization
         'msgpack>=1.0.3',