Tweak import placement to fix startup

i.e. per Continuum needs

rattail_tempmon/commands.py

@@ -32,8 +32,6 @@ import logging
 from rattail import commands
 from rattail.time import localtime, make_utc
 
-from rattail_tempmon.db import Session as TempmonSession, model as tempmon
-
 
 log = logging.getLogger(__name__)
 
@@ -62,6 +60,8 @@ class PurgeTempmon(commands.Subcommand):
                             "occur, but rollback (abort) the transaction at the end.")
 
     def run(self, args):
+        from rattail_tempmon.db import Session as TempmonSession, model as tempmon
+
         cutoff = localtime(self.config).date() - datetime.timedelta(days=args.keep)
         cutoff = localtime(self.config, datetime.datetime.combine(cutoff, datetime.time(0)))
         session = TempmonSession()