Set version to v6.4.0 (#2896)

Author: moisses89

safe_transaction_service/__init__.py

@@ -1,5 +1,5 @@
 # SPDX-License-Identifier: FSL-1.1-MIT
-__version__ = "6.3.0"
+__version__ = "6.4.0"
 __version_info__ = tuple(
     int(num) if num.isdigit() else num
     for num in __version__.replace("-", ".", 1).split(".")