man: consistently use #!/usr/bin/python

Some files were using that, others weren't. Since those are user-facing
docs, we should use the nice clean form, not the workaround for strange
systems that we use in other places.

man/90-rearrange-path.py

@@ -1,4 +1,4 @@
-#!/usr/bin/env python3
+#!/usr/bin/python
 # SPDX-License-Identifier: MIT-0
 
 """

man/notify-selfcontained-example.py

@@ -1,4 +1,4 @@
-#!/usr/bin/env python3
+#!/usr/bin/python
 # SPDX-License-Identifier: MIT-0
 #
 # Implement the systemd notify protocol without external dependencies.