flake8 - misc flake8 errs.

* unused variables
* variables in loops overshadowing imports
* excepts with no defined exc type (tried to avoid `except Exception` too)
* history mapper is still too complex
* default variables should never be mutable

app/history_meta.py

@@ -33,7 +33,7 @@ def _is_versioning_col(col):
     return "version_meta" in col.info
 
 
-def _history_mapper(local_mapper):
+def _history_mapper(local_mapper):  # noqa (C901 too complex)
     cls = local_mapper.class_
 
     # set the "active_history" flag