Add return_on_result flag, to return immediately when a result is received

[noamraph]

• I have added an entry to CHANGELOG.md

Summary of changes

When I run a command and wait for the response, many times I don't want to wait for the entire timeout. So I added a flag return_on_result, which returns immediately when a response of type 'result' is received.

Notes:

• I made this False by default to preserve backwards compatibility.
• I added this flag to IoManager.write() as well, together with timeout_sec and raise_error_on_timeout which are also passed to get_gdb_response(). I added it after raise_error_on_timeout and before read_response, to keep it with the other get_gdb_response() flags. However, this will break code that sets read_response as a positional argument (meaning: iomanager.write('-cmd', timeout, raise_error_on_timeout, read_response). I leave it up to you to decide if you prefer better backwards compatibility or a more logical ordering of arguments.
• The first two commits were just to make nox happy. The second commit might actually have some significance - it seems that .pyc files not in the root directory could be included in sdists.

Test plan

I added an automated test which sets the return_on_result flag and makes sure it returns fast. You can see it fail by replacing True with False in test_gdbcontroller.py:

    responses = gdbmi.write(["-rubbish"], return_on_result=True)