Merged the slinkp-datetime-200007 branch: fix the DateTime(anotherDateTime) constructor to preserve timezones.

'unrestrictedTraverse' (apparent NameError).

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o Also, use tagged release of ZODB-based externals.

Base the default location of softwarehome on Zope2, which is guaranteed not to be a namespace but always located inside the Zope2 software home. This fixes Zope2 breaking on systems which uses namespace packages for Products.*

Revert c81408 HelpSys changes, it didn't make much of a difference. Making the whole help system optional would be a better approach.

Revert ISO8601 interpretation to previously broken behaviour (assume GMT rather than local time as per spec)

(culprit from the last refactoring)

* Add support to timezone aware datetime conversion
* Correct capitalization of Brazil/DeNoronha

All previous timezone names are tested against the new pytz based timezones.
This test shows that the following zones changed when pytz support was added
(i.e. not in this commit):

t1 = time.mktime(datetime(2002, 1, 1).timetuple())
t2 = time.mktime(datetime(2002, 7, 1).timetuple())
expected_failures = [ # zone.info(t1)     newzone.info(t1)     zone.info(t2)     newzone.info(t2)
    'Jamaica',        # (-18000, 0, 'EST') (-18000, 0, 'EST') (-14400, 1, 'EDT') (-18000, 0, 'EST')
    'Turkey',         # (10800, 0, 'EET') (7200, 0, 'EET') (14400, 1, 'EET DST') (10800, 1, 'EEST')
    'Mexico/BajaSur', # (-25200, 0, 'MST') (-25200, 0, 'MST') (-25200, 0, 'MST') (-21600, 1, 'MDT')
    'Mexico/General', # (-21600, 0, 'CST') (-21600, 0, 'CST') (-21600, 0, 'CST') (-18000, 1, 'CDT')
    'Canada/Yukon',   # (-32400, 0, 'YST') (-28800, 0, 'PST') (-28800, 1, 'YDT') (-25200, 1, 'PDT')
    'Brazil/West',    # (-10800, 1, 'WDT') (-14400, 0, 'AMT') (-14400, 0, 'WST') (-14400, 0, 'AMT')
    'Brazil/Acre',    # (-14400, 1, 'ADT') (-18000, 0, 'ACT') (-18000, 0, 'AST') (-18000, 0, 'ACT')
    ]

The most likely explanation for this is that the old database was out of date.