Add --allow-untyped-globals flag that avoids partial type errors

[msullivan]

A major cause of false positives when checking new codebases is
unannotated top level containers and Nones.

Add a flag that suppresses both the "needs type annotation" errors and
the assigning of NoneTyp to variables at the "toplevel" (not when
nested in functions). To avoid throwing away too much type
information, we give more precise types in these situations than
previously, filling in the arguments to the type with Any.

Closes #5639.

I don't love the flag name and would be happy to change it. This just adds an individual flag. Grouping into a --strictness=0 configuration or something we should spin off as another bug, I think.

[gvanrossum]

I'm curious why this doesn't pass:

class C:
    a = []  # Error here
    def f(self) -> None:
        reveal_type(self.a)

This still gives an error "Need type annotation for 'a'" on line 2.

[msullivan]

Because member checking has a totally different partial type path apparently, which I'm now going to merge.

[Michael0x2a]

Just a few thoughts about how we're categorizing this flag:

1. We should add this flag to the config file docs somewhere -- probably as a per-module option?
2. One idea is maybe to group this under the "untyped definitions and calls" section and name the flag something like --allow-untyped-globals. (I'm also not happy with this name, but starting the flag with words like "allow/disallow" feels a little more consistent with what the other flags are doing.)

[msullivan]

Re categorization:
The strictness group currently includes only the meta-option --strict, which doesn't seem like the right place for this.
"Untyped definitions and calls" is the closest fit, and I nearly lumped it in there, but it seemed like something of an outlier still. I'd be happy to move it back in there.

[gvanrossum]

I still think it belongs better in strictness_group. This looks as follows currently:

Other strictness checks:
  --strict                  Strict mode; enables the following flags: --warn-
                            unused-configs, --disallow-subclassing-any,
                            --disallow-untyped-calls, --disallow-untyped-defs,
                            --disallow-incomplete-defs, --check-untyped-defs,
                            --disallow-untyped-decorators, --no-implicit-
                            optional, --warn-redundant-casts, --warn-unused-
                            ignores, --warn-return-any

[gvanrossum]

Yay!

[gvanrossum]

(Note that the flag ended up being renamed to --allow-untyped-globals, but the commit message wasn't adjusted. :-( )