I have a Python program that runs on my Windows 7 computer which communicates with a Raspberry Pi over the internet through a port I opened with a Port Forwarding rule on my internet modem. 
I am concerned about a hacker getting through that open port and causing problems. 
My question is: 
Is there a way to password protect that port so anyone who tries to access that port is required to enter the correct password to get through to my Raspberry Pi?
If not, what other ways could I protect that open port?
Any help/advice is much appreciated. Thanks in advance.
You cannot password protect a port. That concept is several layers up in the network stack and not something regular internet gear has anything to do with.
You'll have to add authentication at the service/application layer. Meaning, your Pi will have to demand authentication. Whether that's possible or not depends on what's running on it.
If that's not available, you'll need an intermediary. Either you set up a proxy in front of the Pi that can handle authentication; or you set up a VPN server instead of a simple forwarded port, which would put the authentication at the point of network access.
Port Forwarding Security