Fenics install with spack

I have come across a new error when installing fenics with spack: fenics install fails due to cmake python ply issue · Issue #25877 · spack/spack · GitHub
This seems like a similar issue to Installation issue: fenics fails to configure due to missing py-ply · Issue #13397 · spack/spack · GitHub.

I am installing spack with the following command:
spack install fenics~hdf5 ^openmpi+pmi schedulers=slurm

I run into cmake issues at the final step. Specifically it cannot locate py-ply and site.py

1 error found in build log:
     147      Ignoring EXACT since no version is requested.
     148    This warning is for project developers.  Use -Wno-dev to suppress it.
     150    -- Found PythonLibs: /cm/shared/apps/spack-pkgs/linux-centos7-zen/gcc-8.2.0/python-3.8.6-u7px4ssttiqjhc6q2arrww4wqceck7nk/lib/libpython3.8.so (found versio
            n "3.8.6")
     151    ImportError: No module named site
     152    -- Could NOT find PY_ply (missing: PY_PLY_path)
  >> 153    CMake Error at CMakeLists.txt:438 (message):
     154      Required Python module 'ply' (http://www.dabeaz.com/ply/) could not be
     155      found.  Install ply or set DOLFIN_ENABLE_PYTHON to false.
     158    -- Configuring incomplete, errors occurred!
     159    See also "/home/apal/.spack_root/stage/spack-stage-fenics-2016.1.0-mgupwbojaxav4wuo63yfu6gv7oiy5jdn/spack-build-mgupwbo/CMakeFiles/CMakeOutput.log".

See build log for details:

I have verified that py-ply is in PYTHONPATH. I have explicitly spack-loaded python3 and py-ply.

[root@L01 tarballs]# spack find --loaded
==> 18 installed packages
-- linux-centos7-zen / gcc@8.2.0 --------------------------------
bzip2@1.0.8   gdbm@1.18.1   libbsd@0.10.0  libiconv@1.16  libxml2@2.9.10  openssl@1.1.1h  python@3.8.6  sqlite@3.33.0  xz@5.2.5
expat@2.2.10  gettext@0.21  libffi@3.3     libuuid@1.0.3  ncurses@6.2     py-ply@3.11     readline@8.0  tar@1.32       zlib@1.2.11
[root@L01 tarballs]#
[root@L01 tarballs]# python -c "import re, ply, site; print(re.compile('/__init__.py.*').sub('',ply.__file__))"                                                         /cm/shared/apps/spack-pkgs/linux-centos7-zen/gcc-8.2.0/py-ply-3.11-fpyplf44xgseqlpml5vxvs4r7bxv4c53/lib/python3.8/site-packages/ply
[root@L01 tarballs]# python -c "import re, ply, site; print(re.compile('/__init__.py.*').sub('',site.__file__))"
[root@L01 tarballs]# sed 's/:/\n/g' <<< "$PYTHONPATH"

Any ideas how to circumvent this issue? Thank you for your attention. Regards.