argparse needs special handling of bokeh args for bokeh 2.4.3 or later

[rekado]

When building with a slightly more recent version of bokeh (2.4.3 instead of 2.1.0) bulkvis.py needs to handle the Serve.args specially, e.g. with code like this:

from bokeh.command.subcommand import Argument
from bokeh.util.dataclasses import entries

...

    if isinstance(opts, Argument):
      opts = dict(entries(opts))

This way the use of add_argument will work even though the Serve.args are wrapped in bokeh's Argument class.