Add whitespace around -> annotating operator (#809)

* Test for whitespace around -> operator

Tests will pass after fixing issue #803

* Require whitespace around -> operator

Closes: #803

* Move tests to correct cases

* Whitelist python3 only tests

* Fix whitespace test errors

Huge thanks to @asottile!

* Address code review

Pushing this directly to run full testsuite on travis

* :bug:Change error code to space around bitwise operator E227

* Check for -> annotation only in py3.5+

* Skip tests meant for higher versions of python

* Move type annotation tests to python3.5 testsuite

Type annotations were first introduced in PEP 484,https://www.python.org/dev/peps/pep-0484/ implemented in python3.5

* Shorten line skipping tests by version

* Replace test skipping logic

As requested in code review

* Run formatting to avoid long lines
3 files changed