Ignore __pycache__ directory (#16836)

.gitignore

@@ -4,6 +4,7 @@
 /var/spack/environments
 /var/spack/repos/*/index.yaml
 /var/spack/repos/*/lock
+__pycache__/
 *.pyc
 /opt
 *~
@@ -36,4 +37,4 @@ lib/spack/spack/test/.cache
 
 # VSCode files
 .vscode
-.devcontainer
\ No newline at end of file
+.devcontainer