CI/COMPAT: PyUnicode_FromUnicode is deprecated failure on 3.9 CI
The 3.9-dev build started failing (eg https://travis-ci.org/github/pandas-dev/pandas/jobs/700233890):

```
    pandas/_libs/parsers.c: In function ‘__Pyx_decode_c_string’:
    pandas/_libs/parsers.c:56645:9: error: ‘PyUnicode_FromUnicode’ is deprecated [-Werror=deprecated-declarations]
             return PyUnicode_FromUnicode(NULL, 0);
             ^~~~~~
    In file included from /opt/python/3.9-dev/include/python3.9/unicodeobject.h:1026:0,
                     from /opt/python/3.9-dev/include/python3.9/Python.h:97,
                     from pandas/_libs/parsers.c:57:
    /opt/python/3.9-dev/include/python3.9/cpython/unicodeobject.h:551:42: note: declared here
     Py_DEPRECATED(3.3) PyAPI_FUNC(PyObject*) PyUnicode_FromUnicode(
                                              ^~~~~~~~~~~~~~~~~~~~~
    cc1: all warnings being treated as errors
```

See https://docs.python.org/3.9/c-api/unicode.html#c.PyUnicode_FromUnicode