lib/plugin/plugin.c: do not use G_MODULE_BIND_LOCAL for Python plugin provider

Using g_module_open() with the G_MODULE_BIND_LOCAL flag when loading the
Python plugin provider makes it impossible for _native_bt.so (the Python
module, part of the `bt2` package, generated by SWIG) to find libpython
symbols because the Python plugin provider is linked with libpython, not
_native_bt.so.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>