setup.py: don't import rd6006

[cstrahan]

This makes it possible to install under poetry (and similar).

Before this change, you'd get an error like so:

PackageInfoError

Unable to determine package info for path: /home/cstrahan/src/cal-riden-psu/../rd6006

Fallback egg_info generation failed.

Command ['/tmp/tmp30wom7ih/.venv/bin/python', 'setup.py', 'egg_info'] errored with the following return code 1, and output:
Traceback (most recent call last):
  File "/home/cstrahan/src/rd6006/setup.py", line 13, in <module>
    import rd6006
  File "/home/cstrahan/src/rd6006/rd6006/__init__.py", line 1, in <module>
    from .rd6006 import *
  File "/home/cstrahan/src/rd6006/rd6006/rd6006.py", line 1, in <module>
    import minimalmodbus
ModuleNotFoundError: No module named 'minimalmodbus'

at ~/.asdf/installs/poetry/1.1.14/lib/poetry/inspection/info.py:500 in _pep517_metadata
    496│                 try:
    497│                     venv.run_python("setup.py", "egg_info")
    498│                     return cls.from_metadata(path)
    499│                 except EnvCommandError as fbe:
  → 500│                     raise PackageInfoError(
    501│                         path, "Fallback egg_info generation failed.", fbe
    502│                     )
    503│                 finally:
    504│                     os.chdir(cwd.as_posix())

This commit resolves that.

See https://packaging.python.org/en/latest/guides/single-sourcing-package-version/