Modernize Syntax for python 3

[lucaswiman]

This PR removes some superfluous syntax that is no longer needed in python 3:

• u"strings" to "strings"
• super(MyClass, self).method(...) to "super().method(...)

The former are literally no-ops in that the exact same bytecode is produced now with or without u-annotation on a string. The latter is usually equivalent, though not always. There is one case in this codebase which is not equivalent and I left it as-is.

[lucaswiman]

Other modernizations to consider:

• Renaming _unicode_members.
• Replacing .assertFoo syntax with ordinary assertions.