Support Python 3.8

[neiljp]

This PR primarily adds a Python 3.8 test entry in Travis and claims support for installation with it via setup.py.

We no longer need special options to test Python 3.7 (or 3.8), so I've removed this too.

NOTE: Pending a later release of the zulip package, I'm keeping this as a draft for now, since while the tests run fine, the current released zulip API version doesn't support 3.8. I've tested this is the case using a docker image.

[LeifW]

Trying to run on python 3.8, after I login, it crashes with:

DEBUG:urllib3.connectionpool:Starting new HTTPS connection (1): %20https:443
DEBUG:urllib3.connectionpool:Starting new HTTPS connection (1): inner-product.zulipchat.com:443
DEBUG:urllib3.connectionpool:https://inner-product.zulipchat.com:443 "POST /api/v1/fetch_api_key HTTP/1.1" 200 None
INFO:root:

module 'platform' has no attribute 'linux_distribution'


ERROR:root:module 'platform' has no attribute 'linux_distribution'
Traceback (most recent call last):
  File "/usr/lib/python3.8/site-packages/zulipterminal/cli/run.py", line 241, in main
    Controller(zuliprc_path,
  File "/usr/lib/python3.8/site-packages/zulipterminal/core.py", line 40, in __init__
    self.model = Model(self)
  File "/usr/lib/python3.8/site-packages/zulipterminal/model.py", line 109, in __init__
    self._update_initial_data()
  File "/usr/lib/python3.8/site-packages/zulipterminal/model.py", line 440, in _update_initial_data
    results = {
  File "/usr/lib/python3.8/site-packages/zulipterminal/model.py", line 441, in <dictcomp>
    name: self.exception_safe_result(future)
  File "/usr/lib/python3.8/site-packages/zulipterminal/model.py", line 386, in exception_safe_result
    return future.result()
  File "/usr/lib/python3.8/concurrent/futures/_base.py", line 432, in result
    return self.__get_result()
  File "/usr/lib/python3.8/concurrent/futures/_base.py", line 388, in __get_result
    raise self._exception
  File "/usr/lib/python3.8/concurrent/futures/thread.py", line 57, in run
    result = self.fn(*self.args, **self.kwargs)
  File "/usr/lib/python3.8/site-packages/zulipterminal/model.py", line 353, in get_messages
    response = self.client.get_messages(message_filters=request)
  File "/usr/lib/python3.8/site-packages/zulip/__init__.py", line 703, in get_messages
    return self.call_endpoint(
  File "/usr/lib/python3.8/site-packages/zulip/__init__.py", line 623, in call_endpoint
    return self.do_api_query(marshalled_request, versioned_url, method=method,
  File "/usr/lib/python3.8/site-packages/zulip/__init__.py", line 505, in do_api_query
    self.ensure_session()
  File "/usr/lib/python3.8/site-packages/zulip/__init__.py", line 450, in ensure_session
    session.headers.update({"User-agent": self.get_user_agent()})
  File "/usr/lib/python3.8/site-packages/zulip/__init__.py", line 466, in get_user_agent
    vendor, vendor_version, dummy = platform.linux_distribution()
AttributeError: module 'platform' has no attribute 'linux_distribution'

This says that's been removed in Python 3.8: pymssql/pymssql#586

[neiljp]

@LeifW The issue you are seeing is within the zulip library; there is a fix in that repo and a release of that is pending right now.

[LeifW]

Found zulip/python-zulip-api#530

[neiljp]

After a brief confusion over whether pipenv was breaking things, I'm merging this now so that we can get things tested - it seems OK in CI 👍

[neiljp]

@LeifW Please let us know if the latest development version has any 3.8-related issues for you here, or in a new issue :)