PYTHON-3167 Fix mockupdb tests for estimated_document_count

[ShaneHarvey]

No description provided.

[ShaneHarvey]

Fixes this mockupdb failure:

 [2022/05/03 02:19:00.604] FAIL: test_estimated_document_count (test_mixed_version_sharded.TestMixedVersionSharded)
 [2022/05/03 02:19:00.604] ----------------------------------------------------------------------
 [2022/05/03 02:19:00.604] Traceback (most recent call last):
 [2022/05/03 02:19:00.604]   File "/data/mci/f4b8464dc14af947152548ddabec981b/src/test/mockupdb/test_mixed_version_sharded.py", line 71, in test
 [2022/05/03 02:19:00.604]     upgrade.old if request.server is self.mongos_old else upgrade.new
 [2022/05/03 02:19:00.604]   File "/data/mci/f4b8464dc14af947152548ddabec981b/src/mockuptests/lib/python3.6/site-packages/mockupdb/__init__.py", line 427, in assert_matches
 [2022/05/03 02:19:00.604]     raise AssertionError('%r does not match %r' % (self, matcher))
 [2022/05/03 02:19:00.604] AssertionError: OpMsg({"count": "collection", "$db": "db"}, namespace="db") does not match Matcher(OpMsg({"aggregate": "collection"}, namespace="db"))

[blink1073]

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