PR: Update to released version of Spyder 6

[jitseniesen]

Description of Changes

• Use released version of Spyder 6 as requirement
• Test against both release version and current master in CI tests
• Bump required Python version to 3.8

Affirmation

By submitting this Pull Request or typing my (user)name below,
I affirm the Developer Certificate of Origin
with respect to all commits and content included in this PR,
and understand I am releasing the same under Spyder's MIT (Expat) license.

I certify the above statement is true and correct:
Jitse Niesen

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 60.86%. Comparing base (ce5897d) to head (7af837b).

Additional details and impacted files

see 7 files with indirect coverage changes