Add python@3.8 handling to virtualenv_install_with_resources

[bayandin]

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Since we have python@3.8 formula, we need virtualenv_install_with_resources to handle it correctly.
Currently for python@3.8 it uses python2.7 which is not what we want.

Another possible solution is explicitly specifying python3.8 in all formulae during the migration:

-    virtualenv_install_with_resources
+    virtualenv_install_with_resources using: "python3.8"

But I still prefer to change formulae as less as possible.

Ref: Homebrew/homebrew-core#47326