In tests, `sys.stderr.write` does not return the number of characters written

Within tests, `sys.stderr.write` does not return the number of characters written as `write` should.
This originates in `EncodedFile.write` which returns None, missing a return statement.

A minimal test (needs to be run as a test by pytest):

```
def test_stderr():
    assert sys.stderr.write("A") > 0
```

I am using pytest 5.2.1 but the code in master is the same.