Provide a default version in `setup.py`

[gabe-l-hart]

Describe the bug

When running setup.py to install locally, the RELEASE_VERSION env var is currently required. If not set, an AssertionError will be raised. Instead, it should simply use a default value like "0.0.0".

Sample Code

python setup.py install

Expected behavior

The module should exit completely and install the local copy of aconfig

Observed behavior

Traceback (most recent call last):                                                                                             
  File "setup.py", line 20, in <module>                                                                   
    assert version is not None, "Must set RELEASE_VERSION"                                                                     
AssertionError: Must set RELEASE_VERSION