Remove event deprecations strategy (for release 0.4.0)

[tlambert03]

This PR removes the event.py module that was mediating the deprecation strategy introduced in #253. This will make magicgui less sensitive to changes in private attributes over in psygnal. The deprecation strategy has been in place for 4 months.

This is a breaking change, so the next release will be 0.4.0.

[codecov]

Codecov Report

Merging #368 (b20f889) into main (c4f563e) will decrease coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #368      +/-   ##
==========================================
- Coverage   88.30%   88.28%   -0.02%     
==========================================
  Files          30       30              
  Lines        3616     3612       -4     
==========================================
- Hits         3193     3189       -4     
  Misses        423      423              

Impacted Files	Coverage Δ
magicgui/backends/_qtpy/widgets.py	86.00% <100.00%> (-0.08%)	⬇️
magicgui/widgets/_bases/button_widget.py	100.00% <100.00%> (ø)
magicgui/widgets/_bases/container_widget.py	92.85% <100.00%> (ø)
magicgui/widgets/_bases/value_widget.py	96.82% <100.00%> (ø)
magicgui/widgets/_bases/widget.py	87.15% <100.00%> (ø)
magicgui/widgets/_function_gui.py	93.99% <100.00%> (ø)

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[aeisenbarth]

These days I noticed that the events.py broke psygnal's connect_setattr when used in MagicGUIs. The attribute setter receives an Event instead of the new attribute value. This is because _new_callback does not contain True for the setattr callback. I was just about to create a patch and then discovered this recent PR.

I then ran my test case with this PR and it also fixes connect_setattr!
Just mentioning that I'm looking forward to the release.

[tlambert03]

Thanks for taking the time to let me know @aeisenbarth! I think we're pretty much in the "safe" zone for merging and releasing this with 0.4.0... likely in a couple weeks or so.