Move renderer optional imports into function calls

[marthacryan]

There are some optional imports in the _renderers.py file in plotly/io that could slow down startup time when the packages aren't present. See #4823 (comment)

[alexcjohnson]

To be clear, the big drain is if these packages are installed in the system but are not imported for any purpose other than us reading their version numbers. In that case we should wait to import them until we actually need that version number.

[gvwilson]

@marthacryan is this small enough that we can sneak it into 6.0 or should it be 6.1? (reluctant to keep adding "little things" to the current release, but if this gives us a performance boost…)