Remove author_email from our Python packaging metadata

[kinow]

These changes close #3713

author_email is one of the keys available for pip's packaging metadata. But it is not mandatory (see https://github.com/pypa/pip/blob/50aca38eafde71ac0314d14246f7ebf043695f5e/src/pip/_vendor/distlib/metadata.py#L620 for the mandatory ones—couldn't find in their docs, had to locate in the code 😢 )

The author_email requires a valid RFC-822 address. Since we are not using Google Groups, and Discourse does not offer an e-mail address, I think the best option is to simply remove it.

If users want to get in contact, they will need to visit our project URL (which is in the packaging metadata) and find how to reach us.

Requirements check-list

• I have read CONTRIBUTING.md and added my name as a Code Contributor.
• Contains logically grouped changes (else tidy your branch by rebase).
• Does not contain off-topic changes (use other PRs for other changes).
• Does not need tests (why? packaging only).
• Appropriate change log entry included.
• No documentation update required.
• No dependency changes. (checked the conda recipe, no mention of google groups 👍 )

[kinow]

One review should be enough. Don't think CI result matters 👍

Logged this issue during the previous release when I found the Google Groups e-mail. So better to fix now before we need to release the next alpha/beta version.

[hjoliver]

thanks @kinow