* Added pyproject.toml
This adds the build requirements metadata to the repo, which can be used
with any build tools that implement PEP 517 and PEP 518 (e.g. pip, tox).
It is no longer necessary to have the build dependencies installed when
installing from source.
* Add python_requires for 2.7, 3.4+
This directive specifies in the build metadata which version of CPython
is supported by this version of spaCy, which pip will take into account
when determining what version to download. This will allow you to safely
drop old versions of Python without `pip install spaCy` breaking for those
versions.
* Add Python 3.7 to the trove classifiers
