actually use cython language_level=3 + fix issue!

jnius/jnius_conversion.pxi

@@ -525,7 +525,7 @@ cdef jstring convert_pystr_to_java(JNIEnv *j_env, unicode py_uni) except NULL:
     py_bytes = py_uni.encode('utf-16')
     # skip byte-order mark
     buff = (<char *>py_bytes) + sizeof(jchar)
-    j_strlen = len(py_bytes) / sizeof(jchar) - 1
+    j_strlen = int(len(py_bytes) / sizeof(jchar) - 1)
     j_str = j_env[0].NewString(j_env, <jchar *>buff, j_strlen)
 
     if j_str == NULL:

setup.py

@@ -98,22 +98,12 @@ compile_native_invocation_handler(*get_possible_homes(PLATFORM))
 # generate the config.pxi
 with open(join(dirname(__file__), 'jnius', 'config.pxi'), 'w') as fd:
     fd.write('DEF JNIUS_PLATFORM = {0!r}\n\n'.format(PLATFORM))
-    if not PY2:
-        fd.write('# cython: language_level=3\n\n')
     fd.write('DEF JNIUS_PYTHON3 = True\n\n')
-    else:
-        fd.write('# cython: language_level=2\n\n')
-        fd.write('DEF JNIUS_PYTHON3 = False\n\n')
 
 # pop setup.py from included files in the installed package
 SETUP_KWARGS['py_modules'].remove('setup')
 
-# create the extension
-setup(
-    cmdclass={'build_ext': build_ext},
-    install_requires=INSTALL_REQUIRES,
-    setup_requires=SETUP_REQUIRES,
-    ext_modules=[
+ext_modules = [
     Extension(
         'jnius', [join('jnius', x) for x in FILES],
         libraries=get_libraries(PLATFORM),
@@ -121,7 +111,18 @@ setup(
         include_dirs=get_include_dirs(PLATFORM),
         extra_link_args=EXTRA_LINK_ARGS,
     )
-    ],
+]
+
+for ext_mod in ext_modules:
+    ext_mod.cython_directives = {'language_level': 3}
+
+
+# create the extension
+setup(
+    cmdclass={'build_ext': build_ext},
+    install_requires=INSTALL_REQUIRES,
+    setup_requires=SETUP_REQUIRES,
+    ext_modules=ext_modules,
     extras_require={
         'dev': ['pytest', 'wheel', 'pytest-cov', 'pycodestyle'],
         'ci': ['coveralls', 'pytest-rerunfailures', 'setuptools>=34.4.0'],