Type hint api.py (#3143)

* First batch of type hints

* Some more

* Add type hints to some more transform_methods

* Add type hints to some more transform_methods

* Finish first pass of type hints for public objects

* Move UndefinedType always to last item of Union

* Type hint transform_filter

* Type hint Chart.__init__

* Type hint .facet

* Minor stuff

* Various mypy error fixes

* Fix remaining mypy errors

* Add more core Altair classes to type hints

* Add Parameter hints

* Minor fix

* Exclude IO from __init__.py

* Apply code suggestion

Co-authored-by: Mattijn van Hoek <[email protected]>

* Remove ignore statement which is redundant for new versions of mypy

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Co-authored-by: Mattijn van Hoek <[email protected]>