MAINT: Make pandasSQL_builder private

[gfyoung]

Per discussion with @jorisvandenbossche here.

[codecov-io]

Current coverage is 84.38%

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[gfyoung]

@sinhrks , @jorisvandenbossche , @jreback : any updates on this? Seems relatively trivial to merge.

[jreback]

lgtm. @jorisvandenbossche

[jorisvandenbossche]

Sorry for the delay.
I did a quick search on github, and few people did actually use this in their code (and that were not copies of pandas), so not sure this is worth to change.
I meant we should have made it internal when it was added in 0.14, but not sure if it is worth to change.

[gfyoung]

@jorisvandenbossche : Ah, that's a good point. If the intention was internal, but people are actually using this function for their own pandas engines, I wonder if we should even make it internal at all. It seems like it could be useful for people's own custom usages then.

[jorisvandenbossche]

Yep, then we just let it be :-)

[jorisvandenbossche]

Sorry for my inclarity triggering you to action

[gfyoung]

No worries. Good that we at least investigated.