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: Repeated timeseries plot may result in incorrect kind
Merge pull request #7733 from sinhrks/tsplot_bug

BUG: Repeated timeseries plot may result in incorrect kind
CI: fix typos in readme
BUG: ewma() weights incorrect when some values are missing (GH7543)
Merge pull request #7831 from nathantypanski/typo-correction

CI: fix typos in readme
TST/BUG: html tests not skipping properly if lxml is not installed
BUG: forgot to super the right class
BUG: offsets.apply doesnt preserve nanosecond
Merge pull request #7836 from cpcloud/read-html-test-fix

TST/BUG: html tests not skipping properly if lxml is not installed