Fix supported Python versions in README (#215)

Fixes 35e7ce8

Author: arthurdejong

README.rst

@@ -16,7 +16,7 @@ standard, and most recommended ones. Its features are:
 * Support for exclusive XML canonicalization with inclusive prefixes (`InclusiveNamespaces PrefixList
   <http://www.w3.org/TR/xml-exc-c14n/#def-InclusiveNamespaces-PrefixList>`_, required to verify signatures generated by
   some SAML implementations)
-* Modern Python compatibility (3.6-3.11+ and PyPy)
+* Modern Python compatibility (3.7-3.11+ and PyPy)
 * Well-supported, portable, reliable dependencies: `lxml <https://github.com/lxml/lxml>`_,
   `cryptography <https://github.com/pyca/cryptography>`_, `pyOpenSSL <https://github.com/pyca/pyopenssl>`_
 * Comprehensive testing (including the XMLDSig interoperability suite) and `continuous integration