Fix "library stubs not installed" mypy error

After merging #920,
mypy started to fail with:

'Library stubs not installed for "requests.packages.urllib3.util.retry"'

Looking at the requests source code, it seems that urllib3 is imported
ad-hoc in the packages module, so changing the import to urllib3
directly solved the issue.

I still don't know what caused it originally, since the merge request
was just a bump to the 'packaging' library.

Signed-off-by: Bruno Pimentel <bpimente@redhat.com>

cachito/workers/requests.py

@@ -3,7 +3,7 @@
 
 import requests
 import requests_kerberos
-from requests.packages.urllib3.util.retry import Retry
+from urllib3.util.retry import Retry
 
 from cachito.workers.config import get_worker_config