ci: Update codecov/codecov-action to v2.

v1 will stop working starting from 2022-02-01. [1]

[1] https://github.com/codecov/codecov-action/blob/260aa3b4b2f265b8578bc0e721e33ebf8ff53313/README.md

Signed-off-by: Michael Seifert <[email protected]>

Author: seifertm

.github/workflows/main.yml

@@ -42,7 +42,7 @@ jobs:
 
       - name: "Upload coverage to Codecov"
         if: "contains(env.USING_COVERAGE, matrix.python-version)"
-        uses: "codecov/codecov-action@v1"
+        uses: "codecov/codecov-action@v2"
         with:
           fail_ci_if_error: true