Fixed #8791 add contextmanager to HDFStore

instead of having to use

with get_store('file.h5') as store:
    store.keys()

now we can do

with HDFStore('file.h5') as store:
    store.keys()
BLD: add 3.4 slow tester
DOC: typos in categorical.rst
Merge branch '8791' of https://github.com/vikram/pandas into vikram-8791
Fix groupby().transform() example in cookbook.rst
BUG: Bug in using a pd.Grouper(key=...) with no level/axis or level only (GH8795, GH8866)
Merge pull request #9007 from SteveSimmons/fix-8944-cookbook-transform

Fix groupby().transform() example in cookbook.rst
Merge pull request #9008 from jreback/grouper

BUG: Bug in using a pd.Grouper(key=...) with no level/axis or level only (GH8795, GH8866)
PERF: add exact kw to to_datetime to enable faster regex format parsing for datetimes (GH8904)
BUG: fix GH8989 to parse nanoseconds with %f format