For tests this makes sure that if one test fails, it won't make following
tests fail just because the next test will fail trying to lock test database.

For regular code (demo_zbigarray.py) this is also a good thing to do -
to always close the database irregardless of whether an exception was
raised before program reached end of main.

Pygolang becomes regular - not test only - dependency. Being regular
dependency is currently required only by demo_zbigarray.py, but it will
be also used in upcoming wcfs, so adding pygolang into wendelin.core
dependencies aligns with the plan.

dbclose now uses defer almost everywhere - there are still few places in
tests, where one test function is opening/closing test database multiple
times - those were not (yet ?) converted.