BUGFIX: use axes unit information in ConnectionPatch

[story645]

This PR lets connection patch use the convertor info from the axes that's already being passed in for "data" to support unitized data. I think the original behavior is a bug b/ the original code called the artist convertors but they didn't do anything b/c the artist convertors are looking up the artist axes which doesn't exist b/c connection patch is a figure level artist.

Modifies string convertor to return a scaler when input is only one value b/c that's what the connection patch transform function expects. This is inspired by the datetime convertor, which already behaves this way.

Updated summary based on @QuLogic's review to reflect current implementation/behavior.

This PR is motivated by the following code, which on main draws the respective plots but crashes at rendering the connection patch because it doesn't know about the convertors:

fig, (ax1, ax2) = plt.subplots(nrows=2, figsize=(10, 5))
x = pd.Timestamp('2017-01-01T12')
ax1.axvline(x)
y = "test test"
ax2.axhline(y)
arr = mpatches.ConnectionPatch((x, 0), (0, y), coordsA='data', coordsB='data', axesA=ax1, axesB=ax2)
fig.add_artist(arr)

With the changes in this PR, the ConnectionPatch now renders:

PR checklist

• [n/a] "closes #0000" is in the body of the PR description to link the related issue
• new and changed code is tested
• [n/a] Plotting related features are demonstrated in an example
• [n/a] New Features and API Changes are noted with a directive and release note
• [n/a] Documentation complies with general and docstring guidelines

[QuLogic]

If we want to use an explicit Axes, do we need to go through Artist.convert_xunits (with the addition of the axes parameter), or instead call Axes.xaxis.convert_units ourselves with the correct axes?

[story645]

🤷‍♀️ I did it this way to keep the consistency w/ the other artists but I think your suggestion is more straightforward so I'll make the changes in the morning if there aren't objections.

[story645]

Can this get milestoned to the next micro release since the functionality it's implementing was supposed to exist (going by the existing do nothing calls out to the convertors)?

Only reason I didn't is b/c that milestone isn't listed.