PR: Upgrade all links to use HTTPS where available, and fix link targets when broken/redirected

[CAM-Gerlach]

Pull Request Checklist

• Read and followed this repo's Contributing Guidelines
• Based your PR on the latest version of the correct branch (master or 3.x)
• Followed PEP8 for code style
• Ensured your pull request hasn't eliminated unrelated blank lines/spaces,
  modified the spyder/defaults directory, or added new icons/assets
• Described your changes and the motivation for them below

Description of Changes

Upgrades all URLs to use HTTPS where available, to follow modern best practices, being conservative about skipping any that could potentially have side effects (i.e. any in tests, functional code, non-standard URLs, those not user-accessible or in a non-parsed string, etc;). All links were manually tested before and after conversion, and a number of broken, outdated or redirected ones were updated where possible (or, in rare cases where no valid copy existed online and the link was merely for reference rather than a citation or something functional, removed).

[pep8speaks]

Hello @CAM-Gerlach! Thanks for submitting the PR.

• In the file spyder/__init__.py, following are the PEP8 issues :

Line 35:14: E221 multiple spaces before operator

[ccordoba12]

It looks good, thanks @CAM-Gerlach!

I only have one question: did you verify that all links you changed work in https?

[CAM-Gerlach]

@ccordoba12 Yup!

All links were manually tested before and after conversion

As also mentioned, I didn't touch

any that could potentially have side effects (i.e. any in tests, functional code, non-standard URLs, those not user-accessible or in a non-parsed string, etc;)

along with XML schema/namespaces and the like.