Syncs all the readmes to indicate Python 3.6 is the lowest
supported version.
Change commentary around scons-dev mailing list.
Signed-off-by: Mats Wichmann <mats@linux.com>
Comb out all code that supported earlier versions of Python. Most such
code is in snippets of only a few lines and can be identified by having
a Python version string in it. Such snippets add up; this combing pass
probably got rid of over 500 lines of code.