fix typing for python 3.8

[sutt]

Observed behavior: Install from pypi (pip install bolt11) succeeds, but bolt11 decode fails with TypeError: 'type' object is not subscriptable in utils.py on line:

def verify_hrp(hrp: str) -> tuple[str, Optional[MilliSatoshi]]:

This occurs because subscripting the built-in tuple type with type hints is only supported in Python 3.9 and later. The type hint should be changed to Tuple from the typing module, which is supported in Python 3.8 and later.

Since the pypi package lists support as >=3.8.1, this PR adds a simple way to bring support to 3.8. After applying the fix blot11 decode works.

On 3.8 - it fails

Python 3.8.10 (default, Nov 14 2022, 12:59:47) 
[GCC 9.4.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import sys
>>> sys.version
'3.8.10 (default, Nov 14 2022, 12:59:47) \n[GCC 9.4.0]'
>>> tuple[str, int]
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: 'type' object is not subscriptable

On 3.9 - it succeeds

Python 3.9.7 (default, Sep 16 2021, 13:09:58) 
[GCC 7.5.0] :: Anaconda, Inc. on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import sys
>>> sys.version
'3.9.7 (default, Sep 16 2021, 13:09:58) \n[GCC 7.5.0]'
>>> tuple[str, int]
tuple[str, int]

[dni]

support for python 3.8 will be dropped in the future, fine for now! thanks!

[dni]

ci failing formatting