remove `__doc__ =` and fix tweak doc diffs
Merge pull request #653 from Metallicow/switch-example-correction

remove `__doc__ =` and fix tweak doc diffs
Replace assert(0) with a self-documenting TODO string
unix: Add setjmp-based GC register helper implementation.

As I suspected for a long time, for x86, register helper doesn't really make
any difference - there's simply not enough register to keep anything in
them for any prolonged time. Anything gets pushed on stack anyway. So, on
x86, uPy passed all tests even with empty reg helper. So, this setjmp
implementation goes as "untested".
Merge pull request #660 from Rosuav/assert0

Replace assert(0) with a self-documenting TODO string
Add PEP 393-flags to strings and stub usage.

The test suite all passes, but nothing has actually been changed.
Remove tests that fail under CPython 3.5

See http://bugs.python.org/issue19995 for rationale. As micropython currently
aims for Python 3.3 compatibility I have not changed behaviour, but this
change allows the test suite to be run against a newer Python without having
spurious failures.
Fix a couple of compilation failures in MICROPY_EMIT_CPYTHON mode

Bugs introduced by the PEP 393 flags introduction, easily caught