native_ds_store now takes advantage of Pythons ability to decode binary
plists (since 3.4), so we can drop its biplist dependency.
The call to biplist.Data() in custom_dsstore doesn't seem to do anything,
and from what I can tell can just be removed.
Ubuntu 16.04 "xenial xerus" does not come with Python 2.x by default.
It is possible to install a python-2.7 package, but this has its own
problem: no `python` or `python2` symlink (see #7717).
This fixes the following scripts to work with python 3:
- `make check` (bctest,py, bitcoin-util-test.py)
- `make translate` (extract_strings_qt.py)
- `make symbols-check` (symbol-check.py)
- `make security-check` (security-check.py)
Explicitly call the python commands using $(PYTHON) instead
of relying on the interpreter line at the top of the scripts.
fanquake