[devtools/test] duplicate CubicWebServerTC tests for CubicWebWsgiTC

[devtools/test] fix bytes vs text in test_base_url Needed for python3 with the next changeset

[devtools] skip CubicWebServerTC on python3 People should migrate to wsgi if they're going to use CubicWeb on python3.

[web/test] switch unittest_web from twisted to wsgi And make it pass on python3 while at it.

[devtools] add CubicWebWsgiTC Similar to CubicWebServerTC, except using wsgiref instead of twisted as an http server.

[web/test] return the hash of uploaded files in FileUploadTC instead of their contents json must be unicode, which doesn't go well with arbitrary file contents. Compute the file hash instead, as we know the hexdigest is (ascii) text.

[web/test] open() as a context manager