Bump version to 1.2.0

Updated the `__version__` in `__init__.py` to reflect the new release version 1.2.0.

src/tzst/__init__.py

@@ -4,7 +4,7 @@ Leveraging cutting-edge Zstandard compression to deliver superior performance,
 security, and reliability.
 """
 
-__version__ = "1.1.1"
+__version__ = "1.2.0"
 
 from .core import (
     TzstArchive,