astropy: 4.0 rc1: Can't disable deprecation errors

On the Debian build, I want/need to disable the interpretation of deprecation warnings as error. This is required, since currently some modules issue deprecation warnings with Python 3.8, and generally they are too strict to have them as failures since they then prevent upgrading some packages. I currently disable the deprecation warnings with a small patch which removes these lines: https://github.com/astropy/astropy/blob/aad16d652d8d3897e7f51dc54590e2974df35564/astropy/conftest.py#L25-L30

This however seems not to disable everything; even with this patch I can’t run the tests (during build time):

I: pybuild base:217: cd '/build/astropy-4.0~rc1/.pybuild/cpython3_3.8/build'; python3.8 -m pytest 
============================= test session starts ==============================
platform linux -- Python 3.8.0+, pytest-4.6.6, py-1.8.0, pluggy-0.13.0
Matplotlib: 3.0.2
Freetype: 2.10.1
rootdir: /build/astropy-4.0~rc1, inifile: setup.cfg
plugins: arraydiff-0.3, remotedata-0.3.2, astropy-header-0.1.1, mpl-0.10, openfiles-0.4.0, doctestplus-0.5.0
collected 0 items / 1 errors

==================================== ERRORS ====================================
________________________ ERROR collecting test session _________________________
/usr/lib/python3/dist-packages/_pytest/config/__init__.py:440: in _importconftest
    return self._conftestpath2mod[conftestpath]
E   KeyError: local('/build/astropy-4.0~rc1/.pybuild/cpython3_3.8/build/astropy/conftest.py')

During handling of the above exception, another exception occurred:
/usr/lib/python3/dist-packages/py/_path/common.py:377: in visit
    for x in Visitor(fil, rec, ignore, bf, sort).gen(self):
/usr/lib/python3/dist-packages/py/_path/common.py:418: in gen
    dirs = self.optsort([p for p in entries
/usr/lib/python3/dist-packages/py/_path/common.py:419: in <listcomp>
    if p.check(dir=1) and (rec is None or rec(p))])
/usr/lib/python3/dist-packages/_pytest/main.py:667: in _recurse
    ihook = self.gethookproxy(dirpath)
/usr/lib/python3/dist-packages/_pytest/main.py:482: in gethookproxy
    my_conftestmodules = pm._getconftestmodules(fspath)
/usr/lib/python3/dist-packages/_pytest/config/__init__.py:424: in _getconftestmodules
    mod = self._importconftest(conftestpath.realpath())
/usr/lib/python3/dist-packages/_pytest/config/__init__.py:474: in _importconftest
    self.consider_conftest(mod)
/usr/lib/python3/dist-packages/_pytest/config/__init__.py:527: in consider_conftest
    self.register(conftestmodule, name=conftestmodule.__file__)
/usr/lib/python3/dist-packages/_pytest/config/__init__.py:321: in register
    ret = super(PytestPluginManager, self).register(plugin, name)
/usr/lib/python3/dist-packages/pluggy/manager.py:126: in register
    hook._maybe_apply_history(hookimpl)
/usr/lib/python3/dist-packages/pluggy/hooks.py:333: in _maybe_apply_history
    res = self._hookexec(self, [method], kwargs)
/usr/lib/python3/dist-packages/pluggy/manager.py:92: in _hookexec
    return self._inner_hookexec(hook, methods, kwargs)
/usr/lib/python3/dist-packages/pluggy/manager.py:83: in <lambda>
    self._inner_hookexec = lambda hook, methods, kwargs: hook.multicall(
astropy/conftest.py:35: in pytest_configure
    matplotlibrc_cache.update(matplotlib.rcParams)
/usr/lib/python3/dist-packages/matplotlib/__init__.py:886: in __getitem__
    cbook.warn_deprecated(
/usr/lib/python3/dist-packages/matplotlib/cbook/deprecation.py:112: in warn_deprecated
    warnings.warn(message, category, stacklevel=2)
E   matplotlib.cbook.deprecation.MatplotlibDeprecationWarning: 
E   examples.directory is deprecated; in the future, examples will be found relative to the 'datapath' directory.
!!!!!!!!!!!!!!!!!!! Interrupted: 1 errors during collection !!!!!!!!!!!!!!!!!!!!
=========================== 1 error in 0.20 seconds ============================
E: pybuild pybuild:341: test: plugin distutils failed with: exit code=2: cd '/build/astropy-4.0~rc1/.pybuild/cpython3_3.8/build'; python3.8 -m pytest 
dh_auto_test: pybuild --test --test-pytest -i python{version} -p "3.8 3.7" returned exit code 13

I am not sure whether I did something wrong; however I don’t see how I could disable them otherwise. I also must admit that I don’t understand the warning. I also don’t understand the key error on top. Matplotlib version is 3.0.2, built for both Python 3.8 and 3.7.