ddtrace 0.53.1 on Python PyPI

Fixed the support for Celery workers that fork sub-processes with Python 3.6 and earlier versions.
Fixes cases in which the test.status tag of a test span from pytest would be missing because pytest_runtest_makereport hook is not run, like when pytest has an internal error.
Pin protobuf version to <3.18 for Python <=3.5 due to support being dropped.