Set cython language level to 3

Otherwise it fails to convert unicode to 'str':
```
Error compiling Cython file:
------------------------------------------------------------
...
    if metadata_request_timeout is None:
        return stmt
    ms = int(metadata_request_timeout / datetime.timedelta(milliseconds=1))
    if ms == 0:
        return stmt
    return f"{stmt} USING TIMEOUT {ms}ms"
           ^
------------------------------------------------------------

cassandra/util.py:1813:11: Cannot convert Unicode string to 'str' implicitly. This is not portable and requires explicit encoding.
```

Author: dkropachev

setup.py

@@ -325,10 +325,14 @@ def _setup_extensions(self):
                                extra_compile_args=compile_args)
                         for m in cython_candidates],
                     nthreads=build_concurrency,
-                    exclude_failures=True))
+                    compiler_directives={'language_level': 3},
+                ))
 
-                self.extensions.extend(cythonize(NoPatchExtension("*", ["cassandra/*.pyx"], extra_compile_args=compile_args),
-                                                 nthreads=build_concurrency))
+                self.extensions.extend(cythonize(
+                    NoPatchExtension("*", ["cassandra/*.pyx"], extra_compile_args=compile_args),
+                    nthreads=build_concurrency,
+                    compiler_directives={'language_level': 3},
+                ))
             except Exception:
                 sys.stderr.write("Failed to cythonize one or more modules. These will not be compiled as extensions (optional).\n")