Add stacklevel to deprecation warnings for argument name change

[XuehaiPan]

Changes:

1. Add * to function signature to mark the deprecated argument poll_intervall as keyword only.

2. Add stacklevel=2 for deprecation warnings, show the file and lineno for the user's code rather than this module (filelock).

   From Fix misspelled keyword argument poll_interval for method acquire #119 (comment):

   I don't know where it's coming from (maybe virtualenv?) but I'm getting a lot of warnings