Fix build issues by adding pyproject.toml

[cshimmin]

Fixes gatagat/lap#38.

Previously, it was not always possible to install lap by simply listing it as a dependency e.g. in a requirements.txt file. This is because the setup.py build system depends on numpy.distutils, and so numpy has to be installed before setup.py could run successfully. This is not always guaranteed even if doing something like pip install numpy lap.

By using pyproject.toml, we can specify that numpy>=1.10.1 and cython>=0.21 are requirements for the build step. These will be installed in a build-only environment during the setup. Numpy is also required at runtime, so it is now listed as a dependency as well.