fix support for ipywidgets 8 and remove max pin

[kecnry]

Description

This pull request attempts to support ipywidgets 8 change in .widgets becoming a private ._active_widgets while still supporting the old syntax.

xref jupyter-widgets/ipywidgets#3122

Checklist for package maintainer(s)

This checklist is meant to remind the package maintainer(s) who will review this pull request of some common things to look for. This list is not exhaustive.

• Are two approvals required? Branch protection rule does not check for the second approval. If a second approval is not necessary, please apply the trivial label.
• Do the proposed changes actually accomplish desired goals? Also manually run the affected example notebooks, if necessary.
• Do the proposed changes follow the STScI Style Guides?
• Are tests added/updated as required? If so, do they follow the STScI Style Guides?
• Are docs added/updated as required? If so, do they follow the STScI Style Guides?
• Did the CI pass? If not, are the failures related?
• Is a change log needed? If yes, is it added to CHANGES.rst?
• Is a milestone set?
• After merge, any internal documentations need updating (e.g., JIRA, Innerspace)?

[codecov]

Codecov Report

Merging #1592 (6899c64) into main (25c351b) will not change coverage.
The diff coverage is 100.00%.

❗ Current head 6899c64 differs from pull request most recent head 7f54448. Consider uploading reports for the commit 7f54448 to get more accurate results

@@           Coverage Diff           @@
##             main    #1592   +/-   ##
=======================================
  Coverage   85.74%   85.74%           
=======================================
  Files          94       94           
  Lines        9290     9290           
=======================================
  Hits         7966     7966           
  Misses       1324     1324           

Impacted Files	Coverage Δ
jdaviz/app.py	91.79% <100.00%> (ø)

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[kecnry]

Let's squash and merge when this is approved (to get rid of the failed attempt from the commit history, but its nice to keep here in the PR so I'll avoid squashing in this branch).

[pllim]

I am approving by proxy. Maybe @havok2063 want to have a look too, just in case?

"... that’s a nice solution! It will not impact performance." -- Mario B. ⭐⭐⭐⭐⭐

[havok2063]

Looks good to me.