py: Fix str.replace for case when arg 0 or 1 is empty string.
tests: Fix tests with sets to print sorted set.
pip-micropython: Require command verb, but restrict to "install" only.

So, pip-micropython can/should be run as normal pip:

    pip-micropython install micropython-unittest
py: Revert change to allocation policy for mp_set_t.

Seems that this fixes all set tests.
tests: Fix few tests which depend on order of elements in set.
Merge branch 'master' of github.com:micropython/micropython
tests: Fix more tests which depend on order of elements in set.
objset: Fix incorrect workaround against mp_set_init() munging alloc size.

No longer needed after recent change which guarantees that mp_set_init()
will allocate exact number of slots requested.
py: Revert revert for allocation policy of set hash table.