Make run_wsgi.py python3 compliant
Changes include:
 - Add __future__ for python2
 - Set "ascii" encoding for TemplateLookup
 - Remove text option for error handling (header is already set to HTML)
 - Break apart exception handling for clarity
 - Check that a file exists before opening it
 - Use `open` not `file` with 'rb' flags
 - Remove unnessary "default" `extensions_map`
 - Python3 style print statement