With the aim of getting consistency, and removing the need to learn an
additional term, replace uses of uPy/uPython with MPy/MicroPython.
Rule of thumb was to use "MPy" abbreviation where "CPy" is used nearby, but
the full word MicroPython otherwise.
This work was funded through GitHub Sponsors.
Signed-off-by: Angus Gratton <angus@redyak.com.au>