objtype: Restore our customized behavior of instance_subscr

[jepler]

For pixelbuf's [] to be able to call _transmit on the Python subclass, we need to do the subscripting operation specially.

[jerryneedell]

I tested this on a Funhouse with tip of main -- the Dotstars are working !! yay!

Press any key to enter the REPL. Use CTRL-D to reload.

Adafruit CircuitPython 7.0.0-alpha.1-930-gd1e17fdcc on 2021-04-29; Adafruit FunHome with ESP32S2
>>> 
>>> import adafruit_dotstar
>>> import board
>>> dot = adafruit_dotstar.DotStar(board.DOTSTAR_CLOCK,board.DOTSTAR_DATA,5,brightness=.3)
>>> dot[0] = 0xff
>>> 

this was failing before the PR as reported in #4682 -- also ran the funhouse_simpletest -- works now

[kattni]

Tested successfully with NeoPixel as well.

[jerryneedell]

Tested neopixel on a qtpy_m0_haxpress and dotstar on raspberry pi pico and funhouse.