Fix slice inference to not always assume List.

[robertwb]

R: @chadrik

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Post-Commit Tests Status (on master branch)

Lang	SDK	Apex	Dataflow	Flink	Gearpump	Samza	Spark
Go		---	---		---	---
Java
Python		---			---	---
XLang	---	---	---		---	---	---

Pre-Commit Tests Status (on master branch)

---	Java	Python	Go	Website
Non-portable
Portable	---		---	---

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[robertwb]

Run Python PreCommit

[chadrik]

Looks good!

This is really powerful stuff. I see now why you were disappointed by the lack of inference from mypy.

[robertwb]

Thanks.

Yes, the inference here is not as powerful in some ways as the whole-program inference that mypy, etc. does, but for passing through types through simple lambdas (something very natural for Beam) it is quite useful.