Make it clearer what is public api

[barisione]

• [x] I have added an entry to CHANGELOG.md

Summary of changes

This PR adds __all__ to all pygdbmi modules to make start imports of pygdbmi better. I'm not a big fan of star imports but I think that using __all__ makes it clearer what is public and what not.

To guide me on what is public and what not I just checked what is documented in the docs. This made me wonder, should IoManager.make_non_blocking be public? I think it may be public by mistake. If you agree I can make it private or, if you don't want to break potential users, we can remove it from the docs and from the star import.

I also moved assert_match to the tests as it's not needed by pygdbmi itself.

Test plan

No new tests needed, but tested by running

$ nox -s tests


$ python3
>>> from pygdbmi.gdbmiparser import *
>>> pprint
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
NameError: name 'pprint' is not defined. Did you mean: 'print'?
>>>

(In previous versions pprint would be imported as a side effect of the start import)