socket: Add _GLOBAL_DEFAULT_TIMEOUT constant.
http.client: Add pristine from CPython 3.3.3.
http.client: uPy doesn't have io.RawIOBase, so skip it.
http.client: Test string by type, not by presense of some method.

MicroPython has both encode() & decode() for both str & bytes, and it's
expensive to have it otherwise.
os: Add fsencode() & fsdecode().