GenericProperty (and therefore Expando) changes str values to bytes (sometimes)

## Expected Behavior

If I set the value of a GenericProperty to a `str`, then read it back from datastore, the value should still always be a `str`. The value I read back should equal the value I put in.

## Actual Behavior

If I set a GenericProperty to the string `'oops'`, then read it back from datastore, the value is now `b'oops'`, so it is not equal to the value I put in the property.
If I set a GenericProperty to the string `'oøps'`, then read it back from datastore, the value is still `'oøps'`. Apparently we are deciding to do different behavior depending on the contents of the string.

The same thing happens in appengine Python 2 incidentally, but it was not a big problem in practice because in Python 2, `'oops' == u'oops'` was `True`. In Python 3 it is a problem :)

## Steps to Reproduce the Problem

```
class Oops(ndb.Model):
generic_prop = ndb.GenericProperty()
string_prop = ndb.StringProperty()

oops = Oops(generic_prop="oøps", string_prop="oøps")
assert "oøps" == oops.generic_prop == oops.string_prop
oops.put()
assert "oøps" == oops.generic_prop == oops.string_prop
oops = oops.key.get()
assert "oøps" == oops.generic_prop == oops.string_prop
oops = oops.key.get(use_cache=False)
assert oops.generic_prop == oops.string_prop # <- Succeeds: both properties are still "oøps"

oops = Oops(generic_prop="oops", string_prop="oops")
assert "oops" == oops.generic_prop == oops.string_prop
oops.put()
assert "oops" == oops.generic_prop == oops.string_prop
oops = oops.key.get()
assert "oops" == oops.generic_prop == oops.string_prop
oops = oops.key.get(use_cache=False)
assert oops.generic_prop == oops.string_prop # <- Fails, because now generic_prop is b'oops'
```

## Specifications

- Version: appengine-python-standard 1.1.3, Python 3.11.4
- Platform: MacOS