Update lockfile; drop Python 3.6 support

Python 3.6 EOLed at the end of 2021, see https://endoflife.date/python.
(pyO3 was refusing to build against 3.6).

Author: David Robertson

Cargo.lock

@@ -5,7 +5,7 @@ version = "0.1.3"
 edition = "2018"
 
 [package.metadata.maturin]
-requires-python = ">=3.6"
+requires-python = ">=3.7"
 project-url = {Source = "https://github.com/matrix-org/rust-synapse-compress-state"}
 classifier = [
     "Development Status :: 4 - Beta",