Add pyproject.toml for Numpy and Cython build dependencies

[altendky]

 ~/r/bandmat   master  venv/bin/pip install .
Processing /home/altendky/repos/bandmat
    ERROR: Command errored out with exit status 1:
     command: /home/altendky/repos/bandmat/venv/bin/python3.8 -c 'import sys, setuptools, tokenize; sys.argv[0] = '"'"'/tmp/pip-req-build-xm52v00g/setup.py'"'"'; __file__='"'"'/tmp/pip-req-build-xm52v00g/setup.py'"'"';f=getattr(tokenize, '"'"'open'"'"', open)(__file__);code=f.read().replace('"'"'\r\n'"'"', '"'"'\n'"'"');f.close();exec(compile(code, __file__, '"'"'exec'"'"'))' egg_info --egg-base /tmp/pip-req-build-xm52v00g/pip-egg-info
         cwd: /tmp/pip-req-build-xm52v00g/
    Complete output (5 lines):
    Traceback (most recent call last):
      File "<string>", line 1, in <module>
      File "/tmp/pip-req-build-xm52v00g/setup.py", line 10, in <module>
        import numpy as np
    ModuleNotFoundError: No module named 'numpy'
    ----------------------------------------
ERROR: Command errored out with exit status 1: python setup.py egg_info Check the logs for full command output.

Then add the pyproject.toml.

 ✘  ~/r/bandmat   master  cat pyproject.toml 
[build-system]
requires = [
    "cython",
    "numpy",
    "setuptools",
    "wheel",
]

 ~/r/bandmat   master  venv/bin/pip install .
Processing /home/altendky/repos/bandmat
  Installing build dependencies ... done
  Getting requirements to build wheel ... done
    Preparing wheel metadata ... done
Collecting scipy>=0.9.0
  Using cached scipy-1.4.1-cp38-cp38-manylinux1_x86_64.whl (26.0 MB)
Collecting numpy>=1.6.1
  Using cached numpy-1.18.1-cp38-cp38-manylinux1_x86_64.whl (20.6 MB)
Building wheels for collected packages: bandmat
  Building wheel for bandmat (PEP 517) ... done
  Created wheel for bandmat: filename=bandmat-0.8.dev1-cp38-cp38-linux_x86_64.whl size=1570166 sha256=78889cc5aad95ef9f3817be92a3a7e283ab57cc783088bc4e081de29774d0741
  Stored in directory: /tmp/pip-ephem-wheel-cache-r69xzul8/wheels/a7/fc/38/c4ce6a501f7a9967d0f23bdfc0f17b051ad4633f360964f3ed
Successfully built bandmat
Installing collected packages: numpy, scipy, bandmat
Successfully installed bandmat-0.8.dev1 numpy-1.18.1 scipy-1.4.1