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.
Replace some Makefile commands with variables in py/mkenv.mk
Merge remote-tracking branch 'upstream/master' into makefile-tweaks
Add tests for sorted() function
and check that sorted(list) produces same output as list.sort()
Modify set tests to print sorted sets directly
instead of creating temporary lists and sorting those in-place
Merge pull request #444 from lurch/test-sorted

Add tests for sorted() function
py: Detect unmatched tripple quote in repl helper.
Merge pull request #445 from lurch/test-sorted-sets

Modify set tests to print sorted sets directly
Merge pull request #439 from lurch/makefile-tweaks

Makefile tweaks
windows: Make it compile with latest changes to py core.
Add uPy welcome message to UNIX and Windows ports; update Teensy port.

Partly addresses issue #154.
Add string formatting support for longlong and mpz.