study lockfile package deprecation situation

[mr-c]

psf/cachecontrol#109

Use the DictCache or other custom backend?

http://cachecontrol.readthedocs.io/en/latest/storage.html#dictcache

Are there other HTTP caching packages that are good?

[nsoranzo]

cachecontrol has just moved from lockfile to filelock in psf/cachecontrol#284 , breaking schema_salad's requirements with cachecontrol 0.12.12

[nsoranzo]

Thanks! Maybe reopen this until we move the pin to CacheControl>=0.13.0 ?