WIP: Type checking with mypy

[xhochy]

First iteration to have type hints. I have found some problems using them, I'll adress those in separate PRs but I will leave the type hints itself here as an FYI and will rebase continuously until we have dropped Py2 so that we can use inline hints.

To make real use of mypy, we would need stubs for the Cython based modules, this depends on someone implementing a pyi generator for Cython, see also cython/cython#3150.

[github-actions]

Thanks for opening a pull request!

Could you open an issue for this pull request on JIRA?
https://issues.apache.org/jira/browse/ARROW

Then could you also rename pull request title in the following format?

ARROW-${JIRA_ID}: [${COMPONENT}] ${SUMMARY}

See also:

• Other pull requests
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[wesm]

Seems like rehabilitating this and adding mypy to our CI would be useful soon

[xhochy]

My main issue holding this back is that I remember that we don't wanted to break 2.7 hard in the source code. Adding type hints and check to pyarrow would be much simpler when we could use 3.6+ syntax.

I'm still from time to time fixing bits that popped up during this work as separate PRs.

[nealrichardson]

We did remove 2.7 support already, so maybe that's not a concern anymore?

[xhochy]

Split this up and made a first round in #6676

[wesm]

Closing this since the work is being divided into separate PRs