Update details of Python 2 support in old versions

Author: ojwb

doc/libstemmer_python_README

@@ -1,9 +1,11 @@
 Snowball stemming library collection for Python
 ===============================================
 
-Python 3 (>= 3.3) is supported.  We no longer actively support Python 2 as
-the Python developers stopped supporting it at the start of 2020.  Snowball
-2.1.0 was the last release to officially support Python 2.
+Python 3 (>= 3.3) is supported.  We no longer support Python 2 as the Python
+developers stopped supporting it at the start of 2020.  Snowball 2.1.0 was the
+last release to officially support Python 2; Snowball 3.0.0 was the last
+release which had the code to support Python 2, but we were no longer testing
+it.
 
 What is Stemming?
 -----------------