On Mongo 3.6+, we don't return 400 'immutable field' error on PATCH and PUT #1243
Description
This only surfaced when, in an attempt to add Python 3.7 to the matrix, we switched to Xenial for our CI runs.
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This only surfaced when, in an attempt to add Python 3.7 to the matrix, we switched to Xenial for our CI runs.