QST: Python 3.9 testing in CI
I use Travis CI to test, and I've tested all of my projects against the upcoming python 3.9 release. The only problem I'm seeing is dependent packages is pandas.

Pandas + 3.9 takes a long time to build and then dies with a compiler error. It also does this when I install cython first, which is the usual workaround when a new version of Python is close to release. Is some not-yet-mainstream cython version needed? I suppose there will be a wheel by the time 3.9.0 comes out in October?

Example CI log: https://travis-ci.com/github/wumpus/paramsurvey/jobs/383337409

```
  pandas/_libs/writers.c:4942:5: error: â€˜_PyUnicode_get_wstr_lengthâ€™ is deprecated [-Werror=deprecated-declarations]
       __pyx_v_l = PyUnicode_GET_SIZE(__pyx_v_val);
       ^
  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/writers.c:33:
  /opt/python/3.9-dev/include/python3.9/cpython/unicodeobject.h:446:26: note: declared here
   static inline Py_ssize_t _PyUnicode_get_wstr_length(PyObject *op) {
                            ^
  pandas/_libs/writers.c:4942:5: error: â€˜PyUnicode_AsUnicodeâ€™ is deprecated [-Werror=deprecated-declarations]
       __pyx_v_l = PyUnicode_GET_SIZE(__pyx_v_val);
       ^
  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/writers.c:33:
  /opt/python/3.9-dev/include/python3.9/cpython/unicodeobject.h:580:45: note: declared here
   Py_DEPRECATED(3.3) PyAPI_FUNC(Py_UNICODE *) PyUnicode_AsUnicode(
                                               ^
  pandas/_libs/writers.c:4942:5: error: â€˜_PyUnicode_get_wstr_lengthâ€™ is deprecated [-Werror=deprecated-declarations]
       __pyx_v_l = PyUnicode_GET_SIZE(__pyx_v_val);
       ^
  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/writers.c:33:
  /opt/python/3.9-dev/include/python3.9/cpython/unicodeobject.h:446:26: note: declared here
   static inline Py_ssize_t _PyUnicode_get_wstr_length(PyObject *op) {
                            ^
  cc1: all warnings being treated as errors
  error: command '/usr/bin/gcc' failed with exit code 1
```

The logfile contains complete version information about the travisci environment.