CLN: make sure that we don't have extraneous imports

[jreback]

xref #9394
xref #7282

numexpr/bottleneck are ok by default

you can check with something like this:

[jreback-~/pandas] python -v -c 'import pandas' > & out
[jreback-~/pandas] grep 'import ' out | more

• matplotlib (not sure if this is possible to remove by default) The converters are set, not so easy to remove.
• io/packers.py imports blosc and possibly other things, CLN: make sure that we don't have extraneous imports #9482
• io/gbq.py import a lot of things (e.g. apiclient/googleapiclient/oauth2client/gflags), CLN: make sure that we don't have extraneous imports #9482
• xlsxwriter this is set in core/config_init.py for the default engine. Not easy to remove.
• html5lib/bs4 (from io/html.py), CLN: make sure that we don't have extraneous imports #9482

These should be done on first usage of the library, and not on general import
like in io/pytables.py (e.g. the _tables()) call.

[jreback]

@cpcloud can you fix the html imports?