Some xc tests fail with libxc5

https://gitlab.com/gpaw/gpaw/-/tags/20.1.0 with a26ade30 (note that CI does not seem to test this change against libxc5 https://gitlab.com/gpaw/gpaw/-/jobs/470606110). I'm unable to test gpaw master due to #265 (closed)

On Fedora 33 libxc5 is the default https://koji.fedoraproject.org/koji/taskinfo?taskID=44230101

python3 -m gpaw test --range=linalg/gemm_complex.py,vdw/potential.py
which: no gpaw-python in (/builddir/build/BUILD/gpaw-20.1.0/python3/tools:/usr/bin:/bin:/usr/sbin:/sbin:/usr/local/sbin)
python-3.8.3rc1          /usr/bin/python3
gpaw-20.1.0              /builddir/build/BUILD/gpaw-20.1.0/python3/gpaw/
ase-3.19.1               /usr/lib/python3.8/site-packages/ase/
numpy-1.18.4             /usr/lib64/python3.8/site-packages/numpy/
scipy-1.4.1              /usr/lib64/python3.8/site-packages/scipy/
libxc-5.0.0              
_gpaw                    /builddir/build/BUILD/gpaw-20.1.0/python3/build_serial/lib.linux-x86_64-3.8/_gpaw.cpython-38-x86_64-linux-gnu.so
parallel                 no
MPI enabled              no
scalapack                no (MPI unavailable)
Elpa                     no (MPI unavailable)
FFTW                     no
libvdwxc                 no
PAW-datasets             1: /usr/local/share/gpaw-setups
                         2: /usr/share/gpaw-setups
Running tests in /tmp/gpaw-test-7mbr1udf
Jobs: 1, Cores: 1, debug-mode: 0
...
xc/xc.py                                      0.012  FAILED! (rank 0,1,2,3)
#############################################################################
RANK 0,1,2,3:
Traceback (most recent call last):
  File "/builddir/build/BUILD/gpaw-20.1.0/python3/gpaw/test/__init__.py", line 672, in run_one
    exec(compile(fd.read(), filename, 'exec'), loc)
  File "/builddir/build/BUILD/gpaw-20.1.0/python3/gpaw/test/xc/xc.py", line 119, in <module>
    equal(abs(d0_xg - d_xg).max(), 0, 2e-8)
  File "/builddir/build/BUILD/gpaw-20.1.0/python3/gpaw/test/__init__.py", line 29, in equal
    raise AssertionError(msg)
AssertionError:  0.040730309550942895 != 0 (error: |0.040730309550942895| > 2e-08)
#############################################################################
xc/XC2.py                                     0.044  FAILED! (rank 0)
#############################################################################
RANK 0:
Traceback (most recent call last):
  File "/builddir/build/BUILD/gpaw-20.1.0/python3/gpaw/test/__init__.py", line 672, in run_one
    exec(compile(fd.read(), filename, 'exec'), loc)
  File "/builddir/build/BUILD/gpaw-20.1.0/python3/gpaw/test/xc/XC2.py", line 67, in <module>
    equal(x, x2, 1e-11)
  File "/builddir/build/BUILD/gpaw-20.1.0/python3/gpaw/test/__init__.py", line 29, in equal
    raise AssertionError(msg)
AssertionError:  -5.823103976200911e-05 != -4.55737652477195e-05 (error: |-1.2657274514289607e-05| > 1e-11)
#############################################################################