set yas-indent-line to 'fixed in python-mode

Following discussion in capitaomorte/yasnippet#506; in particular, @shaleh [writes][s]:

> Based on what I have found so far it looks like setting certain modes to always be in 'fixed' mode is probably the right choice. Very few snippets will work correctly in python.el.

[s]: https://github.com/capitaomorte/yasnippet/issues/506#issuecomment-52719179