furl doesn't type-hint itself as Text

If `furl` were a `typing.Text`, it would work with static type checkers/hinters.

I like that the following works:

```python
url_obj = furl('http://www.google.com')
response = requests.get(url_obj)
```

However, the static checker (I'm using PyRight) complains that `furl.furl.furl` does not match `request.post`'s signature for `Text | bytes`.

I'm newish to Python still, but I think it would require adding the superclass:

```python
from typing import Text

# at https://github.com/gruns/furl/blob/d0bee9a27d7f432b047194a94f64cd4ff0319f6a/furl/furl.py#L1337-L1338
class furl(URLPathCompositionInterface, QueryCompositionInterface,
FragmentCompositionInterface, UnicodeMixin, Text):
```

Then using a factory function named `furl`.

In the meantime, this is my workaround to satisfy the checker:

```python
# in my utils/__init__.py
from typing import Text

from furl import furl as FurlOrig

class Furl(FurlOrig, Text): # bonus: capitalized class name less surprising
pass

def furl(*args, **kwargs) -> Furl:
return Furl(*args, **kwargs)

# usage in other files
from utils import furl

# use `furl` as normal
```