Fixing async syntax for python 3.7

[lexadler]

'async' become reserved keyword in Python 3.7 so variable should be renamed to avoid SyntaxError:

  File "/home/radler/PycharmProjects/ui_testing/env/lib/python3.7/site-packages/pytractor/webdriver.py", line 22, in <module>
    from .mixins import WebDriverMixin
  File "/home/radler/PycharmProjects/ui_testing/env/lib/python3.7/site-packages/pytractor/mixins.py", line 78
    async = kwargs.pop('async', True)
          ^
SyntaxError: invalid syntax

[lexadler]

@RemoNova only pytractor code was fixed and no tests were changed with the first commit. Due to that the build was failed. Now tests are fixed and python 3.7 added to Travis for further testing. Builds was passed in my forked repo so everything is ok now.

[remnov]

@lexadler Yes I saw that.
Unfortunately we need to wait for @kpodl to merge PR.

[A1exKH]

The main change for this PR:
async -> is_async

LGTM.