[electrum] bump the version 5.2.7 and add release notes

Summary: As per title

Test Plan:
```
python setup.py --version
python test_runner.py
./electrum-abc
```

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D14474

RELEASE-NOTES.md

@@ -1,5 +1,17 @@
 # Release notes
 
+## Release 5.2.7
+
+- Fix a bug in the command line argument processing preventing the application
+  from recognizing BIP 21 payment URLs when passed as the only argument. This
+  fixes the `ecash:` MIME type association with Electrum ABC (D14382).
+- Speed up the size estimation for freshly generated transactions by removing
+  the need to serialize them first (D14434, D14448).
+- Slightly improve the performance of some serialization operations for by reducing
+  the number of unnecessary bytes-to-hex conversions (D14455, D14462, D14463, D14464).
+- Support Python 3.12 (D14440).
+
+
 ## Release 5.2.6
 
 - Increase the maximum number of concurrent get_merkle network requests to speed

electrumabc/version.py

@@ -1,7 +1,7 @@
 import re
 
 # version of the client package
-VERSION_TUPLE = (5, 2, 6)
+VERSION_TUPLE = (5, 2, 7)
 PACKAGE_VERSION = ".".join(map(str, VERSION_TUPLE))
 # protocol version requested
 PROTOCOL_VERSION = "1.4"