ENH: do not convert mixed-integer type indexes to datetimeindex
DOC/TST: add test and release notes
Merge pull request #3878 from cpcloud/index-join-date-casting-fix-3877

ENH: do not convert mixed-integer type indexes to datetimeindex
ENH: Add unit keyword to Timestamp and to_datetime to enable passing of

    integers or floats that are in an epoch unit of s, ms, us, ns
    (e.g. unix timestamps or epoch s, with fracional seconds allowed)
    (GH 3540)
TST: disallow slicing a timeseries with floats

TST: manage truediv in py3 for unit comparisons
BUG: make sure that nan/none like values to Timestamp are returned as NaT