Now pypi.org discards a package that does not pass long_description
validation. In our case:

 | The project's long_description has invalid markup which will not be
 | rendered on PyPI. The following syntax errors were detected:
 | line 92: Error: Error in "code-block" directive:
 | maximum 1 argument(s) allowed, 5 supplied.
 |
 | .. code-block:: console
 |    $ python setup.py test

Also added long_description_content_type to eliminate `twine check
dist/*` warning:

 | Checking distribution dist/tarantool-0.6.6.tar.gz: warning:
 | `long_description_content_type` missing.  defaulting to `text/x-rst`.