Update tarantool-python to 0.7.1-1-g8847b8c

This update brings tarantool-python changes from releases 0.6.6[^1],
0.7.0 and 0.7.1 and the unreleased change [1] regarding unification of
the __str__ Response method across Python 2 and Python 3. Look at the
release notes [2] for the list of changes in the connector.

We need [1] to obtain the same test result on different Python versions
for tarantool's box-py/call.test.py test.

[^1]: Except replacing of yaml.load() with yaml.safe_load() in tests,
      which is already here.

[1]: tarantool/tarantool-python#186
[2]: https://github.com/tarantool/tarantool-python/releases

Part of #20
Part of tarantool/tarantool#5652