fixed character set description in docstring (SRE uses Python

fixed character set description in docstring (SRE uses Python
strings, not C strings)

removed USE_PYTHON defines, and related sre.py helpers

skip calling the subx helper if the template is callable.
interestingly enough, this means that

	def callback(m):
	    return literal
	result = pattern.sub(callback, string)

is much faster than

	result = pattern.sub(literal, string)