Fix codecov.yml syntax

It expects `paths` to be a list (though a string had been working previously) but for some reason Codecov will silently ignore mistakes and won't tell you that the file has incorrect syntax.

codecov.yml

@@ -1,25 +1,25 b''
 coverage:
   status:
     patch: off
     project:
       default: false
       library:
         target: auto
-        paths: '!.*/tests/.*'
+        paths: ['!.*/tests/.*']
       tests:
         target: auto
-        paths: '.*/tests/.*'
+        paths: ['.*/tests/.*']
 codecov:
   require_ci_to_pass: false
 
 ignore:
   - IPython/kernel/*
   - IPython/consoleapp.py
   - IPython/core/inputsplitter.py
   - IPython/lib/inputhook*.py
   - IPython/lib/kernel.py
   - IPython/utils/jsonutil.py
   - IPython/utils/localinterfaces.py
   - IPython/utils/log.py
   - IPython/utils/signatures.py
   - IPython/utils/traitlets.py