Docs: Panel.dropna now exists, update docs accordingly.
BUG: Fixed failure in StataReader when reading variable labels in 117 files

Stata's implementation does not match the online dta file format description.
The solution used here is to directly compute the offset rather than reading
it from the dta file.  If Stata fixes their implementation, the original code
can be restored.
closes #7816
ENH: PeriodIndex ops can accept Index
API: rolling_* functions should not shrink window (GH7766)
TST/COMPAT: skip scipy tests if needed (GH7764)
Merge pull request #7818 from bashtage/stata-117-variable-labels

BUG: Fixed failure in StataReader when reading variable labels in 117
DOC: v0.15.0.txt corrections
API/BUG: Bug in passing a DatetimeIndex with a timezone that was not being retained in DataFrame construction from a dict (GH7822)
Merge pull request #7823 from jreback/tz

BUG: Bug in passing a DatetimeIndex with a timezone that was not being retained in Frame construction (GH7822)
Merge pull request #7741 from sinhrks/period_ops

ENH/BUG: DatetimeIndex and PeriodIndex in-place ops behaves incorrectly
Merge pull request #7806 from lexual/docs_panel_has_dropna

Docs: Panel.dropna now exists, update docs accordingly.
BUG: offsets.apply doesnt preserve nanosecond