fix: Remove '<4' from python_requires in packaging guide

As discussed in https://discuss.python.org/t/use-of-less-than-next-major-version-e-g-4-in-python-requires-setup-py/1066
and other places there can be some side effects of adding `<4` to a
library's python_requires. This PR removes it, as requested in the
discuss.python question.

Additional references of problems known to Matthew for posterity:

* Twitter threads:
  - https://twitter.com/HEPfeickert/status/1362170933658738701
  - https://twitter.com/nicholdav/status/1366978826790666240
* GitHub Issues:
  - python-poetry/poetry#3747

source/guides/distributing-packages-using-setuptools.rst

@@ -405,7 +405,7 @@ Python 4 support yet, write::
 If your package is for Python 2.6, 2.7, and all versions of Python 3 starting
 with 3.3, write::
 
-    python_requires='>=2.6, !=3.0.*, !=3.1.*, !=3.2.*, <4',
+    python_requires='>=2.6, !=3.0.*, !=3.1.*, !=3.2.*',
 
 And so on.