Fix setting _selection_completed in SpanSelector when spanselector is initialised using extents

[ericpre]

PR summary

In the following example, if the first click is outside of the span selector, the span selector will be reset on the press event, while it shouldn't. The reason is that when setting extents programmatically after initialisation of the span selector, the attribute _selection_completed should be True, otherwise the span selector will behave as the selection was incomplete!

import matplotlib.pyplot as plt

from matplotlib.widgets import SpanSelector

fig, ax = plt.subplots()
ax.plot(range(40))

span = SpanSelector(
    ax,
    onselect=lambda a, b:None,
    direction="horizontal",
    interactive=True,
    drag_from_anywhere=True,
    ignore_event_outside=True,
)

span.extents = (10, 20)

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