adjusted to use ensure_list to not allocate a new list on py2

See merge request !169

See merge request !167

tutorials.

for the purposes of Wendelin tutorial.

I2C.
Still keep example which simulates same process for Wendelin tutorial.

See merge request nexedi/wendelin!155

See merge request nexedi/wendelin!165

After loading the data from the ZODB, the 'getBucketBy...' methods
now free unused memory. Before this patch, it was the responsibility
of the user to deactivate no longer needed buckets, now this happens
automatically.

Before this patch 'getBucketByIndex' was buggy & would fail with

    AttributeError: 'str' object has no attribute 'value'

But it wasn't recognized, because this method wasn't tested before. This
patch adds a test to the method & fixes it.

See merge request !163

See merge request !162

See merge request nexedi/wendelin!161

This reverts commit 111919f5

As noted by @jm in !160 (comment 199327)
'self.tic() automatically commits first'. So let's drop redundant code
in tests.