DFA._cross_product compares input symbols, fails if one is set and other frozenset

[Tagl]

1. What version were you using? Most recent version (v8.4.0)
2. What were you doing? Taking the intersection of two DFA, one with a set of input symbols and the other with a frozen set of input symbols. In particular I constructed the left hand and right hand side DFAs using the substring, subsequence, of_length and count_mod helper functions.
3. What did you expect would happen? It would work without exceptions
4. What happened instead? An exception of type SymbolMismatchError was thrown

I think we don't want this to be an issue for users to manually take care of and I believe we can expect the input symbols to either be a set or a frozen set so we should handle the 4 possible combinations appropriately.

[eliotwrobson]

Can you post the exact error message you had? I think fixing this will just involve casting both of the sets as frozensets before running the cross product function.

EDIT: Couldn't reproduce this when I wrote a test for it on this PR.

[Tagl]

I'll make a few examples when I'm finished teaching. I double checked that I'm on the latest version.