Mention minimum supported Python version is 3.9

Author: deeplook

README.rst

@@ -77,7 +77,7 @@ Features
 - test entire `W3C SVG test suite`_ after pulling from the internet
 - test all SVG `flags from Wikipedia`_ after pulling from the internet
 - test selected SVG `symbols from Wikipedia`_ after pulling from the net
-- support Python 3.7+ and PyPy3
+- support Python 3.9+ and PyPy3
 
 
 Known limitations