Deprecate Image.show(command="...")

[nulano]

For #4643

Changes proposed in this pull request:

• Deprecate the Image.show(command="...") parameter and remove its documentation, because it hasn't been implemented since the PIL fork, and implementing it is non-trivial.

[nulano]

There was a really odd error on GHA Windows PyPy, unrelated to this change:

__________________ ERROR collecting Tests/test_pdfparser.py ___________________
C:\hostedtoolcache\windows\PyPy\3.6.9\x86\site-packages\py\_path\local.py:701: in pyimport
    __import__(modname)
C:\hostedtoolcache\windows\PyPy\3.6.9\x86\site-packages\_pytest\assertion\rewrite.py:143: in exec_module
    source_stat, co = _rewrite_test(fn, self.config)
C:\hostedtoolcache\windows\PyPy\3.6.9\x86\site-packages\_pytest\assertion\rewrite.py:330: in _rewrite_test
    co = compile(tree, fn, "exec", dont_inherit=True)
E   TypeError: expected unicode, got Call object

[radarhere]

I've created #4695 as an alternative to this, exploring actually implementing command.