Rename cache directory

[mnonnenmacher]

The python-inspector currently creates a cache directory called ".cache/thirdparty". As ".cache" is a very generic name I would prefer to replace it with a name that can be associated with the tool, for example ".python-inspector/thirdparty".

[pombredanne]

We fixed this in #114 wuth a new CACHE_THIRDPARTY_DIR environment variable and also handle the case when the cache cannot be written to in a more graceful way. See https://github.com/nexB/python-inspector/blob/main/CHANGELOG.rst#v094
Feedback welcomed!