Pycurl 7.45.3 breaks our celery workers

[amickan]

Pycurl version 7.45.3 is broken. Import fails with ImportError: pycurl: libcurl link-time ssl backend (gnutls) is different from compile-time ssl backend (openssl)

[jmsmkn]

I could not get a newer version of pycurl to work, it turns out that the dependency has been removed from kombu. I made a PR to reflect that in Celery. If that's all accepted we can remove the direct dependency pinning and test once Celery 5.5 is released and integrated.