fix: avoid overly strict dependency on pyarrow 3.x

[tswast]

Excludes bignumeric_type from all extra

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Fixes #563 🦕

[tswast]

I suspect that the reason it wasn't failing on Kokoro but it was for me is that our Kokoro setup has an older version of pip that didn't have as strict of dependency resolver.

[plamut]

I suppose the assumption is that users who want BIGNUMERIC support will not use "all", but by explicitly specifying the "bignumeric_type" extra?

The change can be breaking for users that decided to use the fairly recently added BIGNUMERIC support through the "all" extra, but that's probably less bad than everyone else on Python3.6 Mac already using the "all" extra, but are now suddenly facing a regression due to a feature they did not voluntarily request.