Update how compatible Python is specified in setup.py

Listing python in the requires field is unusual, and tools either ignore it or sometimes break. There's now a python_requires keyword to express this, and tools make use of the information.

I can't seem to fork this project to make a MR, so I'm putting the change in patch format below. This is the core piece of a commit by Nathan Rosenbloom, who has made a et-xmlfile-2021 fork on PyPI.

Obviously you'd need to upload a new release for this fix to take effect.

diff -r 50973a6de49c -r 8f28d818168e setup.py
--- a/setup.py  Sat Dec 15 16:53:44 2018 +0100
+++ b/setup.py  Sun Mar 14 12:12:02 2021 +0000
@@ -56,9 +56,7 @@
     author_email=__author_email__,
     url=__url__,
     license=__license__,
-    requires=[
-        'python (>=2.6.0)',
-        ],
+    python_requires='>=2.6, !=3.2',
     classifiers=[
                  'Development Status :: 5 - Production/Stable',
                  'Operating System :: MacOS :: MacOS X',