Move Python SPKGs to subfolder `pypi/`, rename to match their PyPI name

[mkoeppe]

After refactoring (#37430 and #36740) gives us more flexibility, we may be able to put them in the subfolder.

• cypari -> pypi/cypari2
• dateutil -> pypi/python-dateutil

The format is intended to match the PURL, see https://peps.python.org/pep-0725/#concrete-package-specification-through-purl, #37250

• Depends on sage --package, sage-get-system-packages: Support PURLs pkg:pypi/DISTRO-NAME, obtain dependencies of wheels from PyPI #37500
• Depends on Refactor bootstrap, bootstrap-conda, m4/sage_spkg_collect.m4, sage-spkg-info through sage --package properties, sage-get-system-packages #37430
• Depends on Refactor bootstrap-conda, m4/sage_spkg_collect.m4 through sage-package; handle dependencies_build #36740

Component: packages: standard

Issue created by migration from https://trac.sagemath.org/ticket/31136

[mkoeppe]

Description changed:

--- 
+++ 
@@ -1,4 +1,4 @@
 - `cypari` -> `cypari2`
-- `datetime` -> `python_dateutil`
+- `dateutil` -> `python_dateutil`
 
 

[mkoeppe]

comment:2

Setting new milestone based on a cursory review of ticket status, priority, and last modification date.