PYTHON-3987 Pin coverage for now (#1384)

Author: blink1073

.evergreen/run-tests.sh

@@ -258,7 +258,7 @@ python -c 'import sys; print(sys.version)'
 # Only cover CPython. PyPy reports suspiciously low coverage.
 PYTHON_IMPL=$($PYTHON -c "import platform; print(platform.python_implementation())")
 if [ -n "$COVERAGE" ] && [ "$PYTHON_IMPL" = "CPython" ]; then
-    python -m pip install pytest-cov
+    python -m pip install pytest-cov "coverage<7.3"
     TEST_ARGS="$TEST_ARGS --cov pymongo --cov-branch --cov-report term-missing:skip-covered"
 fi