cask +python27 also depends on python39

[ryandesign (Ryan Carsten Schmidt)]

cask without a variant selected depends on python39; fine:

$ port info cask
cask @0.8.5_1 (devel)
Variants:             python27

Description:          Cask is a project management tool for Emacs that helps automate the package development cycle; development, dependencies, testing, building, packaging and more. Cask can also be used to manage dependencies for your
                      local Emacs configuration.
Homepage:             https://github.com/cask/cask

Library Dependencies: python39
Platforms:            darwin
License:              GPL-3+
Maintainers:          Email: amake@macports.org, GitHub: amake
                      Policy: openmaintainer

But cask +python27 depends on both python39 and python27; not expected:

$ port info cask +python27
cask @0.8.5_1 (devel)
Variants:             +python27

Description:          Cask is a project management tool for Emacs that helps automate the package development cycle; development, dependencies, testing, building, packaging and more. Cask can also be used to manage dependencies for your
                      local Emacs configuration.
Homepage:             https://github.com/cask/cask

Library Dependencies: python39, python27
Platforms:            darwin
License:              GPL-3+
Maintainers:          Email: amake@macports.org, GitHub: amake
                      Policy: openmaintainer

Why does the port need a python27 variant at all?

If it does need a python27 variant, then it should also have a python39 variant, and the two should be marked conflicting, and the port should ensure that one or the other is selected. This makes it clearer to the user that they are making a choice between two options (radio buttons), rather than a choice between whether or not to have an optional feature (checkbox).

[wyuenho (Jimmy Yuen Ho Wong)]

Python has been added into depends-lib twice it seems. Either we make a variant python39 and default that variant or we can remove python27, or somehow fix the port group so the previous python is removed from depends-lib before appending.

[wyuenho (Jimmy Yuen Ho Wong)]

​https://github.com/macports/macports-ports/pull/10078