Add cop to prevent regexp literals as arguments to assert_matches

[rwstauner]

Add a cop that checks for regular expression literals being passed to assert_raises.

These will not validate the message for the raised exception but will generate a potentially confusing TypeError when the expected exception does not raise.

I didn't see docs for updating config/default.yml, let me know if I need to change that.

Thanks!

---

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[rwstauner]

thanks for the thorough review and all the explanations!

[koic]

Thanks!