Running shebangless Python scripts may hang as /bin/sh runs `import`

As mentioned in https://matrix.to/#/!fny2klHK6fAZn_zfrCzd22UQxWritgFAuTLtN-ZFn_I/$QU6fnHcMogIKEfof8M7aIiP3bKcIv8nnFNFKMony0yc?via=gitter.im&via=matrix.org&via=tchncs.de when executing a Python script that doesn't start with a #!/bin/python line, fish assumes #!/bin/sh. Some Python scripts start with import. The shell executes import, a tool shipped with imagemagick that reads from stdin before doing anything. This caused users to not notice the error immediately.

IIRC fish already disables this logic for *.fish files. Maybe we should do the same for *.py files.