MAM code not compatible with python 3.5

Newly added MAM code uses astimezone on naive date objects, which apparently as been added in Python 3.6.

While I would love to move to newer Python, and Debian buster now comes with Python 3.7, it might help some users who still haven't moved. I'm probably willing to let this go through if it gets annoying though. TDB.

Traceback:

2019-09-10 18:26:43,411 DEBUG:core:Input is readable.
2019-09-10 18:26:43,412 DEBUG:core:Got from keyboard: (['KEY_PPAGE'],)
2019-09-10 18:26:43,414 ERROR:base_events:Exception in callback Core.on_input_readable()
handle: <Handle Core.on_input_readable()>
Traceback (most recent call last):
  File "/usr/lib/python3.5/asyncio/events.py", line 126, in _run
    self._callback(*self._args)
  File "/usr/src/poezio/poezio/core/core.py", line 614, in on_input_readable
    func()
  File "/usr/src/poezio/poezio/core/core.py", line 1429, in scroll_page_up
    if self.tabs.current_tab.on_scroll_up():
  File "/usr/src/poezio/poezio/tabs/basetabs.py", line 922, in on_scroll_up
    return mam.mam_scroll(tab=self, action='scroll')
  File "/usr/src/poezio/poezio/mam.py", line 155, in mam_scroll
    tzone = datetime.now().astimezone().tzinfo
ValueError: astimezone() cannot be applied to a naive datetime