The function PyTokenizer_FromUTF8 from Parser/tokenizer.c had a comment:

    /* XXX: constify members. */

This patch addresses that.

In the tok_state struct:
    * end and start were non-const but could be made const
    * str and input were const but should have been non-const

Changes to support this include:
    * decode_str() now returns a char * since it is allocated.
    * PyTokenizer_FromString() and PyTokenizer_FromUTF8() each creates a
        new char * for an allocate string instead of reusing the input
        const char *.
    * PyTokenizer_Get() and tok_get() now take const char ** arguments.
    * Various local vars are const or non-const accordingly.

I was able to remove five casts that cast away constness.

New changeset 384f3c536dd15ba33ea7e8afb4087ae359d4c12e by Andy Lester in branch 'master':
closes bpo-39721: Fix constness of members of tok_state struct. (GH-18600)
https://github.com/python/cpython/commit/384f3c536dd15ba33ea7e8afb4087ae359d4c12e