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I think that Phillip's proposal is reasonable: when you give
a tzinfo argument to datetimetz.fromtimestamp(), it should
*convert* to the given tzinfo. I think the proper semantics
are specified by this expression:

datetimetz.utcfromtimestamp(stamp).replace(tzinfo=utc).astimezone(tzinfo)

The rule for now() follows from this.

BTW, I think it was a mistake to let astimezone() convert
from/to naive times. It should only be used for conversion
between tz-aware times; conversion from/to naive time should
be done using dt.replace(tzinfo=...).