Stop issuing PURGE requests on stale serial as PyPI requires authentication now

[ichard26]

In #57, we added code to automatically issue PURGE requests to stale endpoints. This was to work around a persistent issue w/ PyPI where caches become routinely stale and unusable. This hack worked for a good while, but now PyPI requires authentication for PURGE requests, flooding the logs with useless 401 errors.

❯ curl -X PURGE -i https://pypi.org/pypi/alchemize/json
HTTP/2 401 
server: Varnish
retry-after: 0
content-type: application/json
fastly-purge-auth-failure: 1
accept-ranges: bytes
date: Tue, 03 Jan 2023 00:03:15 GMT
x-varnish: 2329315070
via: 1.1 varnish
x-served-by: cache-yyz4563-YYZ
content-length: 53

{"msg":"Provided credentials are missing or invalid"}⏎   

See also: #57 (comment)