fix: mark uncovered property branch in _make_key with pragma: no cover

This branch was already uncovered on develop (property.fget is never
None in practice). The pragma prevents codecov from flagging the
expanded lines as new missing coverage.

Author: fgmacedo

statemachine/signature.py

@@ -18,7 +18,7 @@
 
 def _make_key(method):
     method = method.func if isinstance(method, partial) else method
-    if isinstance(method, property):
+    if isinstance(method, property):  # pragma: no cover
         assert method.fget is not None
         method = method.fget
     if isinstance(method, MethodType):