initial setup of durus git clone in bitbucket
replace __len__() of BTree by get_count().

I consider __len__ that might pull gigabytes of Btree data a design error.
This also makes durus better usable in Wing IDE.
move files around to make "setup.py develop" work.
adjust setup.py to added folder level
rename the utest files into test files, for pytest
add conftest.py
started to ported all tests to pytest, first is test_btree. added --slow option to omit slow tests by default
adjusted 8 more test files, continuing...
now all tests are working with pytest.

test_persistent_set was a bit harder because __init__ was used.
Implemented that using @pytest.mark.parametrize
for python3 compatibility, remove iter(keys|values|items) from persistent_dict and btree, if pythonn3 is run.
rewrite of exec file equivalent for python3
adapted a patch that improves packing
adjustment of copyright, required for sdist
Add to changes.txt and update version to 3.9.1, change required python version to 2.6