Enable static type checking with pytype (#298)

* Ignore pytype import errors in google/auth.

These errors are raised because the packages are not required, i.e. not
listed in setup.py. We can't guarantee they'll be installed or that
their pyi files exist in github.com/python/typeshed, so silence
potential errors instead.

This does impact the accuracy of type checking.

* Ignore pytype import errors in transport/.

These imports are not listed in setup.py, because they are optional --
it is assumed the user has them installed if needed. Disabling
import-error for these imports prevents useless errors from pytype.

* Ignore various type errors raised by pytype.

- `__init__.py`: pytype is not aware of `__path__`.
- jwt.py: the pyi file for urllib.unparse is not aware of None. Empty
strings are clearer.

* Add pytype disable comments for scripts/

oauth2client isn't listed as a requirement, so users may not have it
installed.

* Fix lint errors from pytype directives.

* Enable pytype -V3.6.

A few notes:
- Previous commits fixed type errors detected by pytype.
- setup.cfg disables pytype's `pyi-error`. This is necessary due to
incomplete type stubs in https://github.com/python/typeshed.
- This only enables pytype's Python3.6 checks. Python2.7 is supported by
pytype but incomplete type stubs cause spurious type errors.

* Remove pytype directives.

Updates to pytype made these directives unnecessary.

* Move pytype install command.

* Add pytype to tox.

* Remove pytype directives for tox-installed imports.

These imports are handled by setup.py and tox.ini, so they'll be
available when pytype is run under tox.

* Fix lint error.
diff --git a/google/auth/_oauth2client.py b/google/auth/_oauth2client.py
index 71fd7bf..afe7dc4 100644
--- a/google/auth/_oauth2client.py
+++ b/google/auth/_oauth2client.py
@@ -25,6 +25,7 @@
 
 from google.auth import _helpers
 import google.auth.app_engine
+import google.auth.compute_engine
 import google.oauth2.credentials
 import google.oauth2.service_account
 
@@ -37,7 +38,7 @@
         ImportError('oauth2client is not installed.'), caught_exc)
 
 try:
-    import oauth2client.contrib.appengine
+    import oauth2client.contrib.appengine  # pytype: disable=import-error
     _HAS_APPENGINE = True
 except ImportError:
     _HAS_APPENGINE = False