Replace isinstance(foo, str) for py3 support

[Martin Sandve Alnæs]

Replace isinstance(foo, str) with isinstance(foo, string_types), given from six import string_types.

There are 72 occurances across all fenics components according to this grep:

grep 'isinstance(.*,[ ]*str)' `find -name \*.py` | grep -v build | wc

[Martin Sandve Alnæs]

Real programmers use sed.

sed -i 's/isinstance(\(.*\),[ ]*str)/isinstance(\1, string_types)/g' `find -name \*.py`

[Martin Sandve Alnæs]

Fixes in dijitso, ufl, ffc, fiat, dolfin (no needed in instant) are in master and/or next branches.

[Prof Garth Wells]

Can this now be closed?

[Martin Sandve Alnæs]

Found a few more, pushing asap.

[Martin Sandve Alnæs]

pushed to next, buildbots don't seem to work