Python plugin: ERROR: Could not find a version that satisfies the requirement cmake (from versions: none)

I’m trying to build a snap for a pretty complex python program, that uses a ton of libraries including GTK3 and Cython.

I’m getting a message that I believe I can ignore since it says “Resorting to fetching from PyPI”. Is this correct?

The snap is built, so I assume I can ignore it?
(I ask, since there are problems running the program once the snap is installed)

  Installing build dependencies ... error                                      
  ERROR: Command errored out with exit status 1:                               
   command: /root/parts/alpha/install/usr/bin/python3 /root/parts/alpha/install/lib/python3.6/site-packages/pip install --ignore-installed --no-user --prefix /tmp/pip-build-env-1gfa6pf6/overlay --no-warn-script-location --no-binary :none: --only-binary :none: --no-index --find-links /root/parts/alpha/python-packages -- setuptools wheel cmake cython                                               
       cwd: None                                                               
  Complete output (5 lines):                                                   
  Looking in links: /root/parts/alpha/python-packages                          
  Processing /root/parts/alpha/python-packages/setuptools-47.3.1-py3-none-any.whl
  Processing /root/parts/alpha/python-packages/wheel-0.34.2-py2.py3-none-any.whl
  ERROR: Could not find a version that satisfies the requirement cmake (from versions: none)
  ERROR: No matching distribution found for cmake                              
  ----------------------------------------                                     
ERROR: Command errored out with exit status 1: /root/parts/alpha/install/usr/bin/python3 /root/parts/alpha/install/lib/python3.6/site-packages/pip install --ig
nore-installed --no-user --prefix /tmp/pip-build-env-1gfa6pf6/overlay --no-warn-script-location --no-binary :none: --only-binary :none: --no-index --find-links
 /root/parts/alpha/python-packages -- setuptools wheel cmake cython Check the logs for full command output.
Not all dependencies where satisfied with python-packages or requirements. Resorting to fetching from PyPI.