Bug summary

Import of matplotlib leads to KeyError: 'webagg.port' on Slackware64-current.
Seems related to #21684, #21662

During building matplotlib from pypi.org source-tar.gz, patches corresponding to above commits had been applied, giving this
matplotlibrc:

..
## ***************************************************************************
## * BACKENDS *
## ***************************************************************************
## The default backend. If you omit this parameter, the first working
## backend from the following list is used:
## MacOSX QtAgg Gtk4Agg Gtk3Agg TkAgg WxAgg Agg
## Other choices include:
## QtCairo GTK4Cairo GTK3Cairo TkCairo WxCairo Cairo
## Qt5Agg Qt5Cairo Wx # deprecated.
## PS PDF SVG Template
## You can also deploy your own backend outside of Matplotlib by referring to
## the module name (which must be in the PYTHONPATH) as 'module://my_backend'.
#backend: QtAgg
..

Still, also after commenting out #backend: QtAgg in either
~/.config/matplotlib/matplotlibrc
or in
/usr/lib64/python3.9/site-packages/matplotlib/mpl-data/matplotlibrc

results in this KeyError: 'webagg.port'

Code for reproduction

bash-5.1$ python
>>> import matplotlib

Actual outcome

bash-5.1$ python
Python 3.9.9 (main, Nov 20 2021, 19:41:34)
[GCC 11.2.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import matplotlib
Traceback (most recent call last):
File "", line 1, in
File "/usr/lib64/python3.9/site-packages/matplotlib/init.py", line 895, in
defaultParams = rcsetup.defaultParams = {
File "/usr/lib64/python3.9/site-packages/matplotlib/init.py", line 898, in
rcParamsDefault[key]),
File "/usr/lib64/python3.9/site-packages/matplotlib/init.py", line 674, in getitem
return dict.getitem(self, key)
KeyError: 'webagg.port'

Expected outcome

no import error, i.e:

bash-5.1$ python
Python 3.9.9 (main, Nov 20 2021, 19:41:34)
[GCC 11.2.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import matplotlib
>>>

Additional information

With matplotlib-3.4.3 nothing like this.

Operating system

Slackware64-15.0-RC2

Matplotlib Version

3.5.0

Matplotlib Backend

not possible

Python version

3.9.9

Jupyter version

not used

Installation

from source (.tar.gz)