suppress `psy-reg plugin` for now (before `psy-reg==1.5.0` is reeased)

[valeriupredoi]

Description

• Closes #3638
• Link to documentation: tis ere

psyplot=1.5.0 and psy-reg=1.4.0 are incompatible:

(esmvaltool) valeriu@valeriu-PORTEGE-Z30-C:~/ESMValTool$ conda list psy
# packages in environment at /home/valeriu/miniconda3/envs/esmvaltool:
#
# Name                    Version                   Build  Channel
psy-maps                  1.5.0           py311h38be061_1    conda-forge
psy-reg                   1.4.0           py311h38be061_3    conda-forge
psy-simple                1.5.0           py311h38be061_1    conda-forge
psyplot                   1.5.0           py311h38be061_1    conda-forge
(esmvaltool) valeriu@valeriu-PORTEGE-Z30-C:~/ESMValTool$ python esmvaltool/diag_scripts/psyplot_diag.py
WARNING:psyplot.config.rcsetup:Failed to import EntryPoint(name='plugin', value='psy_reg.plugin', group='psyplot')!
WARNING:psyplot:Could not import EntryPoint(name='plugin', value='psy_reg.plugin', group='psyplot')
Traceback (most recent call last):
  File "/home/valeriu/miniconda3/envs/esmvaltool/lib/python3.11/site-packages/psyplot/__init__.py", line 132, in get_versions
    mod = ep.load()
          ^^^^^^^^^
  File "/home/valeriu/miniconda3/envs/esmvaltool/lib/python3.11/site-packages/importlib_metadata/__init__.py", line 184, in load
    module = import_module(match.group('module'))
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/valeriu/miniconda3/envs/esmvaltool/lib/python3.11/importlib/__init__.py", line 126, in import_module
    return _bootstrap._gcd_import(name[level:], package, level)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "<frozen importlib._bootstrap>", line 1204, in _gcd_import
  File "<frozen importlib._bootstrap>", line 1176, in _find_and_load
  File "<frozen importlib._bootstrap>", line 1147, in _find_and_load_unlocked
  File "<frozen importlib._bootstrap>", line 690, in _load_unlocked
  File "<frozen importlib._bootstrap_external>", line 940, in exec_module
  File "<frozen importlib._bootstrap>", line 241, in _call_with_frames_removed
  File "/home/valeriu/miniconda3/envs/esmvaltool/lib/python3.11/site-packages/psy_reg/plugin.py", line 36, in <module>
    from psyplot.compat.pycompat import map, filter
ModuleNotFoundError: No module named 'psyplot.compat'
usage: psyplot_diag.py [-h] [-f] [-i] [-n] [-l {debug,info,warning,error}] filename
psyplot_diag.py: error: the following arguments are required: filename

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