[GRASS-dev] start supporting python3 for GRASS

[Pietro]

Dear devs,

I committed some changes to start supporting python3 (r68348 - r68367)
the commits are a bit messy... but I had some problem with git-svn
.
I've used autopep8 to clean the code from old-style syntax (e.g.
dict.has_key, etc) and then I compared file generated with 2to3 and
manually selected each change that it was needed.

I execute our tests suite, I got a regression in:

{{{
$ python ./temporal/t.connect/testsuite/test_distr_tgis_db_raster3d.py
E
======================================================================
ERROR: setUpClass (__main__.testRaster3dExtraction)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "./temporal/t.connect/testsuite/test_distr_tgis_db_raster3d.py",
line 30, in setUpClass
    cls.runModule("g.mapset", flags="c", mapset=mapset_name)
  File "/home/pietro/docdat/src/gis/grass_git/dist.x86_64-pc-linux-gnu/etc/python/grass/gunittest/case.py",
line 1095, in runModule
    errors=errors)
CalledModuleError: Module run 1 g.mapset ended with error
Process ended with non-zero return code g.mapset(mapset='test3d3',
flags=u'c'). See the following errors:
ERROR: There appears to be an active GRASS session in selected mapset
       <test3d3>


----------------------------------------------------------------------
Ran 0 tests in 6.246s
FAILED (errors=1)
}}}

All the other tests seem to work on my pc.
If you find a new bugs that has been introduced from these changes,
please consider to write a test that highlight the problem before fix
it.

Best regards

Pietro