fix mismatch importlib-metadata with celery imports (relates to celer…

…y/celery#7783)

requirements.txt

@@ -1,10 +1,11 @@
-celery[mongodb]==5.1 # 5.1 fix tasks chain pending indefinitely when a task raises an exception
+celery[mongodb]>=5.2.2
 colander
 cornice>=5
 cornice_swagger>=0.7.0
 dicttoxml  # FIXME: if we need to parse some XML (?)
 # flag --paste breaks for >20 unless using pserve
 gunicorn<20
+importlib-metadata<5; python_version <= "3.7"   # see https://github.com/celery/celery/issues/7783
 lxml>=3.7  # FIXME: if we need to parse some XML (?)
 paste
 pastedeploy
@@ -22,4 +23,3 @@ threddsclient>=0.4.1
 typing_extensions
 watchdog
 webob
-celery>=5.2.2 # not directly required, pinned by Snyk to avoid a vulnerability