[Tooling] Initial implementation of stub checker

[elpekenin]

As per title, just wrote a minimal (and very subject to improvements) script that will compare the actual implementation (.py) against the circuitpython-generated stubs (.pyi)

Note:

• So far, only runs against the BLINKA_OS_AGNOSTIC target
  • I havent tested other targets because i expect them to fail due to lack of some connected hardware, or my computer not being what blinka is being told using the BLINKA_FORCECHIP environment variable
• I dont know whether circuitpython's stubs (which im guessing are generated from the doc-comments on source code) are tested/validated in any way against the actual code that ends up exposed to Python's VM
• Should the requirements license point to me? Im not a lawyer (:

The motivation for this is to find inconsistencies and fix them, such that code is actually portable between devices and not
almost portable because a function signature differs or whatever

[elpekenin]

Some more info in #881