ENH: Improve typing for Period

[bashtage]

• Tests added: Please use assert_type() to assert the type of any return value
  xref ENH: Improve Pandas scalars #383

[Dr-Irv]

@bashtage For this and the other new PRs you created, can you click the "request review" item in the PR when you are ready for me to do the review?

[Dr-Irv]

I know you may have responded to some of these comments in the original PR, but better to restart the conversation here.

[Dr-Irv]

I'm fine with everything here, but would like to change __eq__() and __ne__() as described in my comments.

[bashtage]

I think it is ready. I also added Series[Period] and PeriodSeries for comparisons.

[Dr-Irv]

thanks @bashtage . I think the Literal[False] idea for __eq__() looks good