TST/CLN: Remove makeMixedDataFrame and getMixedTypeDict
thx @mroeschke  Hi. To all people finding this: In a similar spirit like [verlib2](https://pypi.org/project/verlib2/), we rescued the corresponding testing helper methods `makeTimeDataFrame` and `makeMixedDataFrame`, and made them available on behalf of the [pueblo](https://pypi.org/project/pueblo/) package, for all downstream packages which absolutely need them.

- https://github.com/pyveci/pueblo/pull/59
