ReadTheDocs updating x 2.

This commit is the next in a commit chain attempting to rectify the
stunning observation that ReadTheDocs (RTD) has silently refused to bump
@beartype docos *for the past two months.* wut u do, RTD!?!? (*Mingling singles!*)

.github/workflows/python_test.yml

@@ -288,6 +288,6 @@ jobs:
 
       # ..................{ COVERAGE                         }..................
       - name: 'Publishing test coverage to Codecov...'
-        uses: 'codecov/codecov-action@v3'
+        uses: 'codecov/codecov-action@v4'
         with:
           name: "${{ matrix.platform }}-${{ matrix.python-version }}"