Closed issue by paper42 on void-packages repository

https://github.com/void-linux/void-packages/issues/42082

Description:
```
%: echo pkgname=python-gnupg > srcpkgs/python3-gnupg/update
%: ./xbps-src update-check python3-gnupg
...
python3-gnupg-0.4.9 -> python3-gnupg-2.2.0
python3-gnupg-0.4.9 -> python3-gnupg-2.3.0
python3-gnupg-0.4.9 -> python3-gnupg-2.3.1
```
`update-check` strips away the `python-` prefix and tries to check for package updates in https://pypi.org/project/gnupg/ instead of https://pypi.org/project/python-gnupg/. `update-check` shouldn't modify `$pkgname` if it's set in the `update` file.