Replace calls to deprecated asyncio.iscoroutinefunction with calls to
inspect.iscoroutinefunction to prevent the following deprecation
warnings from showing up if running with Python 3.14:
```
/home/user/code/electrum-fork/electrum/util.py:1225: DeprecationWarning: 'asyncio.iscoroutinefunction' is deprecated and slated for removal in Python 3.16; use inspect.iscoroutinefunction() instead
assert asyncio.iscoroutinefunction(func), 'func needs to be a coroutine'
/home/user/code/electrum-fork/electrum/util.py:507: DeprecationWarning: 'asyncio.iscoroutinefunction' is deprecated and slated for removal in Python 3.16; use inspect.iscoroutinefunction() instead
if asyncio.iscoroutinefunction(func):
/home/user/code/electrum-fork/electrum/util.py:1246: DeprecationWarning: 'asyncio.iscoroutinefunction' is deprecated and slated for removal in Python 3.16; use inspect.iscoroutinefunction() instead
assert asyncio.iscoroutinefunction(func), 'func needs to be a coroutine'
/home/user/code/electrum-fork/electrum/lnpeer.py:272: DeprecationWarning: 'asyncio.iscoroutinefunction' is deprecated and slated for removal in Python 3.16; use inspect.iscoroutinefunction() instead
assert asyncio.iscoroutinefunction(func), 'func needs to be a coroutine'
/home/user/code/electrum-fork/electrum/util.py:1225: DeprecationWarning: 'asyncio.iscoroutinefunction' is deprecated and slated for removal in Python 3.16; use inspect.iscoroutinefunction() instead
assert asyncio.iscoroutinefunction(func), 'func needs to be a coroutine'
/home/user/code/electrum-fork/electrum/util.py:507: DeprecationWarning: 'asyncio.iscoroutinefunction' is deprecated and slated for removal in Python 3.16; use inspect.iscoroutinefunction() instead
if asyncio.iscoroutinefunction(func):
```
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