all: Finalise python3 support.

Basically, wrap stdout and stderr whenever they do not have an encoding
with an ascii-encoding writer, and write unicode to stdout & stderr.
wsgi.errors is defined in the reference implementation as being a StringIO,
so follow that.
Stop using argparse.FileType to get rid of python3 "file not closed"
errors.
Also, fix setup access to CHANGES.txt .
Also, fix 2to3 involvement.
Also, replace test.captureStdout with extra tool arguments.