Improvements to --strict-optional[-whitelist]

[gvanrossum]

I'd like to make two changes to --strict-optional-whitelist (abbreviated to --sow below):

• Make multiple occurrences of the flag be cumulatively, so that --sow A --sow B is equivalent to --SOW A B. The motivation for this is to let us put this flag in a script that runs mypy while allowing the script to take additional --sow flags and pass them through.
• Include the flag in the "options" field in the incremental meta json file, so that runs with different settings don't reuse the cache. In fact the json file should probably have three possible values: strict optional off, strict optional on, and strict optional on but excluding this file. (The reason is that the behavior of "strict optional off" really is different from "strict optional on but not in this file".) [UPDATE: This has been implemented]
• A possible third change would be to disallow the form --sow A B C, requiring --sow A --sow B --sow C instead. There are some edge cases to the current behavior that are confusing, e.g. you can't have --sow A immediately followed by the list of files to check.

[JukkaL]

One option is to require a delimiter when providing multiple values with a single --sow argument, for example --sow A,B,C.

[gvanrossum]

Note that people are already using the existing whitelist syntax so I'd like to tread carefully.

[gvanrossum]

@ddfisher Thoughts? Maybe we can retire/deprecate --strict-optional-whitelist in favor of using per-file sections in mypy.ini? E.g.

[mypy]
show_none_errors = True  # same as default
[mypy-foo/*]
show_none_errors = True  # redundant
[mypy-bar/*]
show_none_errors = False

This would not change the behavior of runs without --strict-optional, but for runs with --strict-optional it would suppress the "none" errors in the bar directory.

[ddfisher]

Obsoleted by #3206.