demandimport: explicitly declare `_session` at the module level

The `_session` module level variable is set within a function using the `global`
keyword. This confuses my `test-check-pyflakes.t`. Explicitly declaring the
variable at the top level solves the issue (and seems absolutely reasonable).

hgdemandimport/tracing.py

@@ -12,6 +12,7 b' import os'
 
 _pipe = None
 _checked = False
+_session = 'none'
 
 def _isactive():
     global _pipe, _session, _checked