<!-- Please describe the issue in detail here, and fill in the fields below --> This is a request to add the [constant](https://www.numpy.org/devdocs/reference/constants.html#module-numpy.doc.constants) [`tau`](https://tauday.com/). ### Reproducing code example: <!-- A short code example that reproduces the problem/missing feature. It should be self-contained, i.e., possible to run as-is via 'python myproblem.py' --> ```python import numpy as np >>> np.tau Traceback (most recent call last): File "<stdin>", line 1, in <module> AttributeError: module 'numpy' has no attribute 'tau' >>> np.version.version '1.15.3' ```