Bots: Add support for running packaged bots via entrypoints

[neiljp]

The entrypoints approach seems like it may make it easier to produce third-party bots, and could potentially be extended to be the default way in which zulip_bots accesses built-in bots?

A test is included for the new approach, as well as:

• removal of python 3.4 support and addition of 3.7 to travis (branch failed due to an existing bot requiring lxml with 2.7 or >=3.5)
• addition of ImportError as an alternative to ModuleNotFoundError, which is only introduced in python 3.6
• changes to improve indication to the user, where the bot module is being sourced

[neiljp]

If this is a reasonable approach then we can add support for a 'sample' package, which I've done locally but not included so far.

[timabbott]

I merged the first 2 commits.

[neiljp]

Just adjusted the last commit to take into account 'editable' bot packages, which don't have a version - at least according to entrypoints - so just show the bot module_name details.

[timabbott]

Closing, since we've now merged #708.