i propose to generate these via
```
extensions=['tar.gz', 'tar.xz']
for ext in extensions:
    TEMPLATE_CONSTANTS.append((
        'SOURCE_%s'%ext.replace('.','_').upper(),
        '%(name)s-%(version)s.'+ext,
        "Source .%s tarball"%ext,
    ))
    TEMPLATE_CONSTANTS.append((
        'SOURCELOWER_%s'%ext.replace('.','_').upper(),
        '%(namelower)s-%(version)s.'+ext,
        "Source .%s tarball with lowercase name"%ext,
    ))
```