hg-cpython: fix new occuring TypeError

dd339191f2dc introduced a minor refactoring of error types that highlighted
a fragile error creation mechanism that was in place in the Rust `MixedIndex`.

`PyErr::from_instance` also accepts a Python class, as long as it's an
`Exception` class. Before the aforementioned commit, we never ran into a case
where this duck-typing mechanism failed. We rectify this behavior by doing the
instantiation ourselves.

Differential Revision: https://phab.mercurial-scm.org/D10764

rust/hg-cpython/src/revlog.rs

@@ -469,7 +469,10 b' fn revlog_error(py: Python) -> PyErr {'
         .and_then(|m| m.get(py, "RevlogError"))
     {
         Err(e) => e,
-        Ok(cls) => PyErr::from_instance(py, cls),
+        Ok(cls) => PyErr::from_instance(
+            py,
+            cls.call(py, (py.None(),), None).ok().into_py_object(py),
+        ),
     }
 }