pydrake: Should handle deprecation of implicit conversions to `_int__`

[EricCousineau-TRI]

From @RussTedrake on Slack: https://drakedevelopers.slack.com/archives/C3YB3EV5W/p1619088373041800

Today I encountered

DeprecationWarning: an integer is required (got type pydrake.systems.framework.InputPortIndex).  Implicit conversion to integers using __int__ is deprecated, and may be removed in a future version of Python.
return LinearQuadraticRegulator(plant, context,`

... the LQR method indeed takes an int for (possible) use as an input port index. Of course I can do the cast here, but is this a widespread pattern that we need to fix?

I'm not sure where this arises (CPython / Python, or pybind11), but once we identify it, two possible solutions may just be:

• Use a different spelling, such as __long__
• Workaround it in bindings (either just binding InputPortIndex1 flavor, or add some utilities so that our pybind11 bindings can still do it)

Russ - I assume this was encountered in CPython 3.7 via Google Colab?

[RussTedrake]

actually it was python3.9 on mac.