github/workflows: Switch from Coveralls to Codecov.

As discussed in #7455, Coveralls doesn't work properly anymore, it has
many spurious errors with reduced coverage.

Signed-off-by: Damien George <damien@micropython.org>

.github/workflows/ports_unix.yml

@@ -70,10 +70,12 @@ jobs:
         mkdir -p coverage
         lcov --rc lcov_branch_coverage=1 --directory ports/unix/build-coverage --capture --output-file coverage/lcov.info.all
         lcov --remove coverage/lcov.info.all '*/lib/*' '*/ports/unix/*' '*/utils/*' --output-file coverage/lcov.info
-    - name: Send to coveralls
-      uses: coverallsapp/github-action@master
+    - name: Upload coverage to Codecov
+      uses: codecov/codecov-action@v1
       with:
-        github-token: ${{ secrets.GITHUB_TOKEN }}
+        files: coverage/lcov.info
+        fail_ci_if_error: true
+        verbose: true
     - name: Print failures
       if: failure()
       run: tests/run-tests.py --print-failures

README.md

@@ -1,4 +1,4 @@
-[![CI badge](https://github.com/micropython/micropython/workflows/unix%20port/badge.svg)](https://github.com/micropython/micropython/actions?query=branch%3Amaster+event%3Apush) [![Coverage badge](https://coveralls.io/repos/micropython/micropython/badge.png?branch=master)](https://coveralls.io/r/micropython/micropython?branch=master)
+[![CI badge](https://github.com/micropython/micropython/workflows/unix%20port/badge.svg)](https://github.com/micropython/micropython/actions?query=branch%3Amaster+event%3Apush) [![codecov](https://codecov.io/gh/micropython/micropython/branch/master/graph/badge.svg?token=I92PfD05sD)](https://codecov.io/gh/micropython/micropython)
 
 The MicroPython project
 =======================