autoclass('NSString', ... ) => 'TypeError: expected bytes, str found'

After converting the Print() statements in https://github.com/kivy/pyobjus/blob/master/examples/using_autoclass.py from Python2 to Python3 I deployed it on the iPhone 7 simulator in Xcode ver. 12.4 on MacOS Catalina ver. 10.15.7. The first few statements work fine, but line 17:
```
NSString = autoclass('NSString', load_class_methods=['alloc'], load_instance_methods=['init', 'initWithString:', 'UTF8String'])
```
results in this error:
```
Traceback (most recent call last):
File "/Users/henrik/pyobjus-original-example-ny-ios/YourApp/main.py", line 17, in
File "pyobjus/pyobjus.pyx", line 719, in pyobjus.pyobjus.autoclass
File "pyobjus/pyobjus.pyx", line 556, in pyobjus.pyobjus.class_get_partial_methods
TypeError: expected bytes, str found
2021-03-19 20:57:08.013155+0100 pyobjus-original-example-ny[12926:512737] Application quit abnormally!
2021-03-19 20:57:08.021033+0100 pyobjus-original-example-ny[12926:512737] Leaving
```

Start with examples/using_autoclass.py at line 17, then trace the call through pyobjus.pyx, especially autoclass and class_get_partial_methods. Reproduce the Python 3 failure in the reported iPhone simulator setup and verify that loading NSString with the listed methods completes without the TypeError.

Written by the indexing model from the issue text.