Add license to `setup.py`

[nurdann]

Currently, when the package is checked for license it shows none, e.g.

$ pip install pip-licenses
$ pip-licenses -p python-language-server
 Name                    Version                        License 
 python-language-server  0+untagged.317.ga91a257.dirty  UNKNOWN 

After this change

$ pip-licenses -p python-language-server
 Name                    Version                        License 
 python-language-server  0+untagged.317.ga91a257.dirty  MIT

[palantirtech]

Thanks for your interest in palantir/python-language-server, @nurdann! Before we can accept your pull request, you need to sign our contributor license agreement - just visit https://cla.palantir.com/ and follow the instructions. Once you sign, I'll automatically update this pull request.