numpy build fails on macOS with conda-forge (https://github.com/mkoeppe/sage/runs/542501526)

Traceback (most recent call last):
  File "numpy/core/setup_common.py", line 241, in check_long_double_representation
    ltype = long_double_representation(pyod(obj))
  File "numpy/core/setup_common.py", line 401, in long_double_representation
    raise ValueError("Could not lock sequences (%s)" % saw)
ValueError: Could not lock sequences (None)

This comes from the depth of NumPy's code that tries to determine ABI by inspecting some compiled code (https://github.com/numpy/numpy/blob/master/numpy/core/setup_common.py).

An upgrade to 1.16.6 fixes the build error and works well on other platforms.

This can be tested with #29415:

  tox -e local-conda-forge-macos-standard

Follow-up: #29425 - newer version, py3 only

CC: @isuruf @dimpase @kiwifb @embray

Component: packages: standard

Author: Matthias Koeppe

Branch/Commit: b0c7df6

Reviewer: Isuru Fernando

Issue created by migration from https://trac.sagemath.org/ticket/29429