A quick and dirty way would be:

$ nix run "(import <nixpkgs> {}).python37.withPackages([ pandas statsmodels ])"

This would drop you into a shell with Python 3.7, pandas, and statsmodels installed.

For more complex use cases, you should specify your dependencies in a shell.nix file or write your own nix package.

And then to expand, if you wish to pin one of those packages to a specific rev, the easiest way is to create an "overlay" (it's a Nix design pattern) that you apply to nixpkgs to override whatever version is in your nixpkgs version.