pip allows you to specify, along with package name and version, the expected hash of the downloaded package. If you do that, then pip will download the package, calculate the hash, and check against what you specified. In case of mismatch, pip will refuse to install the package.

PyPI also supports adding GPG signatures alongside packages, but with no trust/verification process to assert "this key really is the key of the person who should be releasing this package", the signature is literally worthless; anyone who could put up a fake package could also generate a signature for it, and you'd have no way of knowing that the key which signed that package shouldn't be trusted for that package.

It is a very very hard problem, and people need to appreciate that.