ci: Add codecov.yml to configure Codecov.io

This change splits up the source code and test code into different blobs
so that the code coverage only considers the source code portion and not
the test suites.

codecov.yml

@@ -0,0 +1,20 @@
+coverage:
+  status:
+    project:
+      production:
+        target: auto
+        flags:
+          - src_code
+
+flags:
+  src_code:
+    paths:
+      - '!**/test_*.c'
+      - '!**/test_*.py'
+      - '**/*.c'
+      - '**/*.h'
+      - '**/*.py'
+  test_code:
+    paths:
+      - '**/test_*.c'
+      - '**/test_*.py'