Add python 3.7 to travis and tox (#14523)

* Add python 3.7 to travis and tox

* Use pyyaml from github

* Don't version constraints

* Fix version tag

* Change to new pyyaml release

* Python 3.7 requires xenial

* Fix namespace detection

* Use correct RegEx type

* Update pexpect to 4.6

* Use correct validation for dictionaries

* Disable Py37 incompatible packages

* Upgrade all pexpect to 4.6

* Add explicit None as default param