First, rename the file from foo.py
to foo
.
Then, move the file to /usr/local/bin/
or /home/user/.local/bin
if the script will only be executed by a single user. Instead, if your script is placed somewhere in the system for example "/path/to/foo", you could add your "/path/to/foo" to the $PATH variable.
After opening a new terminal session. You should be able to execute the script without the "./" and ".py".
By the way "./" means that you want to execute a file in the current working directory. It is always possible to execute a file using a full path of the file, for example "/usr/bin/something_to_run".
Please consider reading about PATH variable here.