Explain the pytest-cov pin

requirements/edx/testing.in

@@ -36,7 +36,7 @@ pyquery                   # jQuery-like API for retrieving fragments of HTML and
 pysqlite                  # DB-API 2.0 interface for SQLite 3.x (used as the relational database for most tests)
 pytest==3.6.3             # Testing framework # Pinned due to https://github.com/pytest-dev/pytest/issues/3749
 pytest-attrib             # Select tests based on attributes
-pytest-cov<2.6            # pytest plugin for measuring code coverage
+pytest-cov<2.6            # pytest plugin for measuring code coverage. # Pinned due to https://openedx.atlassian.net/browse/TE-2731
 pytest-django==3.1.2      # Django support for pytest
 pytest-randomly           # pytest plugin to randomly order tests
 pytest-xdist              # Parallel execution of tests on multiple CPU cores or hosts