When I installed the package I found that I was prompted to enter username and password into settings, even though I have a keychain entry for github.com.
When I inspected the keychain entry for github, I noticed that the name stored in the keychain (presumably automatically via 1Password) was not simply "github.com" but was rather "github.com/mac ()" - when I edited the name to be just github.com then gist.py successfully found the keychain entry and was able to access my github.com account without the need for storing the username and password in settings.
One fix for this might simply be to put this potential issue and the above fix in the installation readme. Of course this depends on 1Password not overwriting my edits (have not looked into the interactions between 1Password and the keychain).
Another fix might be to look for "github.com" as a prefix, but I suspect that might not even be allowed (i.e. you have to search the keychain for an exact match)?
Keychain is deprecated now.