Lazy import IPython [Remove IPython as a required dependency: Part 1]

[dbieber]

IPython is only used in two places:

• inspectutils.py: only uses a single function from IPython (IPython.core.oinspect.Inspector().info), so maybe we could just strip that one function out of IPython or write an equivalent function from scratch. I would want permission from the IPython folks before doing the former.
• interact.py: uses IPython for the --interactive flag, but this should really be optional. You'll notice the unused _EmbedCode function in interact.py. (https://github.com/google/python-fire/blob/master/fire/interact.py#L89) The reason that's there is to eventually enable Fire to be used without IPython as a required dependency.

This task is to lazily import IPython in the two places it's used.

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This is Part 1 of the plan to remove IPython as a required dependency.
The plan is outlined here.

[dbieber]

Done w/ 3db47f8