DEPR: Move rarely used I/O connectors to third party modules
Related to #26804 (and indirectly related to #15862, #15008 if we standardize I/O connectors as part of this)

The recent survey shows that several of our  I/O connectors (Stata, SPSS, SAS and GBQ) are rarely used (from the chart I'd say around 3% of respondents said they used them):

![pandas_survey_io](https://user-images.githubusercontent.com/10058240/64779479-12279b00-d556-11e9-9154-9c8b1c03cb12.png)

While afaik all them are just wrappers around other libraries, and don't contain much code to maintain, I see few main immediate advantages:
- Allow an ecosystem of third-party modules
- Reduce the complexity and time of the CI
- Not commit to the connector API

**I/O ecosystem**

While the number of widely used formats is quite limited, the number of possible I/O connectors is huge. Several new connectors have been requested in the past, like markdown (#11052), avro (#11752), docx (#22518), and there are surely plenty of specialized and internal formats that didn't make sense to propose adding to pandas, but that people would appreciate having and using the same standard API (`pandas.read_*` / `DataFrame.to_*`) we have. And there may also be alternative versions of the connectors we provide (see for example this lightning talk by @dutc dicussing about an unsafe `pandas.read_csv`: https://youtu.be/QkQ5HHEu1b4?t=1554).

**CI**

For the CI, those modules include dependencies that need to be downloaded for every build. We also have files in `pandas/tests/io/data` that eventually we may need to update. For GBQ in particular, we have to store the password for the account we use to test, and those tests fail from time to time for connectivity problems. For the clipboard (not proposing to move to a third-party yet, but is a good candidate too), we require to emulate an X system in our tests (need to check if we finally fixed the tests, but they weren't running for a while because of problems on the CI set up).

And of course our test suite is taking some extra time to test all these connectors.

**API changes**

Being part of our public API, I think we need to commit to the API of those connectors, in the same way as we do with the rest of our API. But, for whatever reason it may make sense to discontinue some of these connectors (e.g. Google releasing a replacement for GBQ...), or make important changes to their parameters, and probably it makes sense to have different release cycles for them.

This also applies to connectors not yet in pandas, but that we may include if we keep the policy of adding to our code base the connectors for all the formats / services we consider relevant.

I know timing is not great, but because of this API commitment, I'd personally remove Stata, SPSS, SAS and GBQ before pandas 1.0.