pyjnius: pin cython version

[T-Dynamos]

As pyjnius build fails on latest cython.

[1/1] Cythonizing jnius/jnius.pyx
warning: jnius/jnius.pyx:102:0: The 'IF' statement is deprecated and will be removed in a future Cython version. Consider using runtime conditions or C macros instead. See https://github.com/cython/cython/issues/4310

Error compiling Cython file:
------------------------------------------------------------
...
            score += 10
            continue

        if r == 'S' or r == 'I':
            if isinstance(arg, int) or (
                    (isinstance(arg, long) and arg < 2147483648)):
                                     ^
------------------------------------------------------------

jnius/jnius_utils.pxi:323:37: undeclared name not builtin: long
Traceback (most recent call last):

[T-Dynamos]

Ci fails because of #3150, which I will fix in following PR.