* master:
  bigfile/py: It is ok to have .fileh_open() as BigFile method

There was an XXX of whether fileh_open should be a BigFile method or
global function. However if it would be a global function it will need
to anyway accept file parameter to indicate which file is opened, and
that in turn suggests that it should be a file method. Remove XXX.

On can just use GOFLAGS env var that is taken by go command itself into
account.

See 69c94fbc and 48eb692f for description of the problem.

* master:
  */tests: Use defer instead of finally
  *: Use defer for dbclose & friends

try/finally was used in a couple of places to save/restore default ZBlk
format setting. Move the restore part close to save with the help of
defer.

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.

* master:
  */tests: Use pytest.raises in modern way

Instead of

	raises(Exception, 'code')

do

	with raises(Exception):
		code

This removes lots of warnings, similar to below example:

	bigfile/tests/test_basic.py::test_basic
	  /home/kirr/src/wendelin/wendelin.core/bigfile/tests/test_basic.py:79: PytestDeprecationWarning: raises(..., 'code(as_a_string)') is deprecated, use the context manager form or use `exec()` directly

	  See https://docs.pytest.org/en/latest/deprecations.html#raises-warns-exec
	    raises(ROAttributeError, "f.blksize = 1") # RO attribute

Else after first failing tests all furhter test can't start because
trying to stat testmntpt/.wcfs gives ENOTCONN.

This allows to make decision to drop a page right after writeout for
wcfs case. If vmas for one fileh were allowed to be mixed - we could not
know whether to drop the page & its memory from RAM or not - even if
there is no vma mapped in !wcfs mode, it could be created later, and by
dropping RAM cahce we hit severe slowness. NOTE: in between transactions
there is usually no vmas mapped, so this case is not artificial.

By requiring that all VMAs are of the same kind under one fileh we avoid
this ambiguity.