Python tests fail with "ValueError: Invalid file object: <_io.BufferedReader name=20>"

Changes to code that is unrelated to Python and Python tests leads to error like

DUNE-INFO: Compiling HierarchicalGrid (new)
Traceback (most recent call last):
  File "test_gf1.py", line 56, in <module>
    gridView = structuredGrid([0,0],[1,1],[10,10])
  File "/builds/core/dune-grid/build-cmake/python/dune/grid/core.py", line 52, in structuredGrid
    return yaspGrid(domain, dimgrid=len(lower))
  File "/builds/core/dune-grid/build-cmake/python/dune/grid/_grids.py", line 38, in yaspGrid
    gridModule = module(includes, typeName, ctor)
  File "/builds/core/dune-grid/build-cmake/python/dune/grid/grid_generator.py", line 348, in module
    module = generator.load(includes, typeName, typeHash, *args, **kwargs)
  File "/duneci/modules/dune-common/build-cmake/python/dune/generator/generator.py", line 169, in load
    return self.post(moduleName, source, postscript, extraCMake)
  File "/duneci/modules/dune-common/build-cmake/python/dune/generator/generator.py", line 121, in post
    module = builder.load(moduleName, source, self.typeName[0], extraCMake)
  File "/duneci/modules/dune-common/build-cmake/python/dune/generator/cmakebuilder.py", line 341, in load
    self.compile(infoTxt=PrintCompiling, target=moduleName)
  File "/duneci/modules/dune-common/build-cmake/python/dune/generator/cmakebuilder.py", line 237, in compile
    logLevel=logging.INFO
  File "/duneci/modules/dune-common/build-cmake/python/dune/generator/cmakebuilder.py", line 215, in callCMake
    _, stderr = cmake.communicate()
  File "/usr/lib/python3.7/subprocess.py", line 939, in communicate
    stdout, stderr = self._communicate(input, endtime, timeout)
  File "/usr/lib/python3.7/subprocess.py", line 1672, in _communicate
    selector.register(self.stdout, selectors.EVENT_READ)
  File "/usr/lib/python3.7/selectors.py", line 352, in register
    key = super().register(fileobj, events, data)
  File "/usr/lib/python3.7/selectors.py", line 238, in register
    key = SelectorKey(fileobj, self._fileobj_lookup(fileobj), events, data)
  File "/usr/lib/python3.7/selectors.py", line 225, in _fileobj_lookup
    return _fileobj_to_fd(fileobj)
  File "/usr/lib/python3.7/selectors.py", line 40, in _fileobj_to_fd
    "{!r}".format(fileobj)) from None
ValueError: Invalid file object: <_io.BufferedReader name=20> 

This happens at least for dune-grid and dune-functions, see for example dune-grid!571 (merged)