switch to using python=3.12 for release testing

doc/release.md

@@ -46,7 +46,7 @@ conda version with `conda --version` and update with `conda update conda`.
 Create the basic build environment:
 
 ```
-mamba create -y -n sourmash-rc python=3.10 pip \
+mamba create -y -n sourmash-rc python=3.12 pip \
     cxx-compiler make twine tox tox-conda rust
 ```