requirements: pin protobuf "<4"

protobuf 4.x introduced breaking changes compared to 3.20.
To adapt, we would have to regenerate paymentrequest_pb2.py, using protoc>=3.19.0, however ubuntu does not have new enough protoc.
Also, unsure if newly generated paymentrequest_pb2.py would be supported by older versions of protobuf.
Best to just wait for things to settle.

```
electrum/__init__.py:20: in <module>
    from .wallet import Wallet
electrum/wallet.py:70: in <module>
    from . import transaction, bitcoin, coinchooser, paymentrequest, ecc, bip32
electrum/paymentrequest.py:37: in <module>
    from . import paymentrequest_pb2 as pb2
electrum/paymentrequest_pb2.py:36: in <module>
    _descriptor.FieldDescriptor(
.tox/py3/lib/python3.9/site-packages/google/protobuf/descriptor.py:560: in __new__
    _message.Message._CheckCalledFromGeneratedFile()
E   TypeError: Descriptors cannot not be created directly.
E   If this call came from a _pb2.py file, your generated code is out of date and must be regenerated with protoc >= 3.19.0.
E   If you cannot immediately regenerate your protos, some other possible workarounds are:
E    1. Downgrade the protobuf package to 3.20.x or lower.
E    2. Set PROTOCOL_BUFFERS_PYTHON_IMPLEMENTATION=python (but this will use pure-Python parsing and will be much slower).
E
E   More information: https://developers.google.com/protocol-buffers/docs/news/2022-05-06#python-updates
```

ceaae1b6a3/CHANGES.txt (L47)

contrib/requirements/requirements.txt

@@ -1,5 +1,5 @@
 qrcode
-protobuf>=3.12
+protobuf>=3.12,<4
 qdarkstyle>=2.7
 aiorpcx>=0.22.0,<0.23
 aiohttp>=3.3.0,<4.0.0