incompatibility with mpl 3.4

[tacaswell]

Describe the issue

matplotlib/matplotlib#19146 made changes to how we encode keyboard events on the js side to ship back to the Python side.

This PR greatly improved the uniformity of handling the keyboard, but think we need to port the js side changes of https://github.com/matplotlib/matplotlib/pull/19146/files?file-filters%5B%5D=.js#diff-179115fdfd653499287a838c855c18302928eccf4d4878acb93da98701246d0d to ipympl or locally restore the old implementation of _handle_key.

[tacaswell]

# %%

%matplotlib widget
import matplotlib.pyplot as plt
import ipywidgets as widgets

# %%

fig, ax = plt.subplots()

# %%
out = []

txt = widgets.Text()

def handler(event):
    out.append(event)
    txt.value = event.key

fig.canvas.mpl_connect('key_press_event', handler)

display(txt)