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