app: add no cover due to false positive

This line is definitely being executed by
test_MetadataSyncJob_success_with_missing_key
but curiously it wasn't being shown as executed in our coverage
report.

Investigating, I discovered the following upstream bug:
https://bugs.python.org/issue2506

The cpython peephole optimizer is causing this line to report
as uncovered (here's a nice blog explaining this optimizer:
https://akaptur.com/blog/2014/08/02/the-cpython-peephole-optimizer-and-you/
this helpfully uses an example (see the section under "Complications arise"
where code is reported as uncovered due to peephole optimizations))

securedrop_client/api_jobs/downloads.py

@@ -64,7 +64,10 @@ def call_api(self, api_client: API, session: Session) -> Any:
                 pub_key = source.key.get('public', None)
                 fingerprint = source.key.get('fingerprint', None)
                 if not pub_key or not fingerprint:
-                    continue
+                    # The below line needs to be excluded from the coverage computation
+                    # as it will show as uncovered due to a cpython compiler optimziation.
+                    # See: https://bugs.python.org/issue2506
+                    continue  # pragma: no cover
                 try:
                     self.gpg.import_key(source.uuid, pub_key, fingerprint)
                 except CryptoError: