$ spack install py-numpy
...
==> Error: ProcessError: Command exited with status 1:
    '/usr/bin/python3.8' '-s' 'setup.py' '--no-user-cfg' 'build'
See build log for details:
  /tmp/permeakra/spack-stage/spack-stage-py-numpy-1.20.3-moeytesgplajh7ebnge5ultezzwfu3h3/spack-build-out.txt

$ spack build-env py-numpy python
Python 3.8.5 (default, Jan 27 2021, 15:41:15) 
[GCC 9.3.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import Cython
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ModuleNotFoundError: No module named 'Cython'

Information on your system

spack debug report

• Spack: 0.16.2-2966-690558f927
• Python: 3.8.5
• Platform: linux-ubuntu20.04-haswell
• Concretizer: original

Forced use of external system python. Spack insists on installing its own Cython. Purging external cython didn't have obvious effect.

Additional information

there were remnants of python2 in the system. Purging them didn't do anything, but maybe I missed smth

General information

• I have run spack debug report and reported the version of Spack/Python/Platform
• I have run spack maintainers <name-of-the-package> and @mentioned any maintainers
• I have uploaded the build log and environment files
• I have searched the issues of this repo and believe this is not a duplicate