Clarify license

[Totktonada]

I would like to resolve several problems here.

The first one is that README.rst states that Tarantool is 'BSD licensed' (which is ambiguous, but correct) and links to the Modified BSD license (which is incorrect). The Modified BSD license is BSD-3-Clause, while Tarantool is licensed under BSD-2-Clause. Check it out: https://github.com/tarantool/tarantool/commits/master/LICENSE. It would be nice to correctly refer BSD-2-Clause and, preferably, without GNU Project comments on it.

Next, tarantool-python license is not defined neither in README.rst, nor in the LICENSE file. I really doubt that anyone meant a license that is different from tarantool's one, so I hope I can assume that this project is licensed under BSD-2-Clause. I propose to add the LICENSE file with BSD-2-Clause license with shared copyright attributed to tarantool-python authors, add the AUTHORS file with the list of all contributors (why not?) and refer to the license file from README.rst explicitly calling the license using the well known SPDX identifier (BSD-2-Clause).

The project refers 'BSD' in setup.py:

tarantool-python/setup.py

Line 72 in 0f95f28

license="BSD",

tarantool-python/setup.py

Line 78 in 0f95f28

"License :: OSI Approved :: BSD License",

It is ambiguous: which one of the BSD family? However, it seems, there is no recommended way to resolve this ambiguity within setup.py: pypa/trove-classifiers#17. I propose to leave it as is until the problem will be resolved on the python packaging infrastructure/tooling side.

[Totktonada]

@coxx I would like to know your opinion on it and, if it would not be too sticky, formal agreement with the statement that the project was always licensed under BSD-2-Clause and had no clear statement about this just by mistake.