Detect and remove features deprecated before 0.20.0
For what I could see, there is still some functionality deprecated before 0.20.0, but which hasn't been removed, as it was missing from #6581.

This includes among others:
* `Categorical.labels`, deprecated in 0.18.0
* The whole `pandas.stats` (rolling_mean...), deprecated in 0.18.0
* `pandas.core.datetools`, deprecated in 0.19.0
* `flavor` parameter of `SQLiteDatabase

I wrote a script that lists the version in which every `FutureWarning` was released (I think it makes sense adding it to pandas).

This is the output for versions older than 0.20.0:

> v0.18.0        3ef82867 600)    pandas/core/categorical.py
> v0.18.0        14cf67fb 132)    pandas/core/resample.py
> v0.18.0        14cf67fb 685)    pandas/core/resample.py
> v0.18.0        14cf67fb 700)    pandas/core/resample.py
> v0.18.0        67730ddf 685)    pandas/core/strings.py
> v0.18.0        3156395c 200)    pandas/stats/moments.py
> v0.18.0        3156395c 240)    pandas/stats/moments.py
> v0.18.1        e4df1ac7 2391)   pandas/util/testing.py
> v0.19.0        3f3839b6 17)     pandas/core/datetools.py
> v0.19.0        3f3839b6 67)     pandas/util/_depr_module.py
> v0.19.0        8acfad34 47)     pandas/io/sql.py
> v0.19.0        8acfad34 56)     pandas/io/sql.py
> v0.19.0        95541952 101)    pandas/util/_depr_module.py
> v0.19.0        b1bfd2fd 939)    pandas/io/parsers.py

Can you please confirm if am I right, and I'll submit a PR?