Mapnik Boost.Python py3k support

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Excellent news - it appears that the GSOC project to bring Boost.Python up to speed with py3k has been accepted, so we'll want to test and upgrade to this as soon as feasible:

http://socghop.appspot.com/student_project/show/google/gsoc2009/boost/t124021999037

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[springmeyer] this project landed in boost 1.41 release, added to trunk here:

https://svn.boost.org/trac/boost/changeset/56305

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[springmeyer] good porting references:

http://wiki.python.org/moin/Py3kExtensionModules

http://docs.python.org/dev/3.0/howto/cporting.html

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[springmeyer] focused link to trunk merge: https://svn.boost.org/trac/boost/changeset/56305/trunk/boost/python/

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[springmeyer] r1518, r1519 adds support for using python scons/scons.py PYTHON=/some/py3k/bin/python

SCons does not work with py3k, but building with py2.x against py3.x should work with the above commit

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[springmeyer] support added in r2239 - will leave open until I get docs up.

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[springmeyer] fixed up in r2240 and r2241

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[springmeyer] reverted test fixes in r 2243 as they were not working - needs another look later on.

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[springmeyer] closing, will deal with tests later in #649