Prepare legacy branch for parking

[brittonsmith]

The recent yt-3.6 release was the last of the 3.x series, so there is no longer any point for the legacy branch to continue to test with the yt master branch. Soon, that will be exclusively for yt-4.0 development and will not work with Trident's legacy branch. This PR decouples Trident legacy branch testing from yt master and instead tests exclusively on the yt-3.6.0 release. I've also made the following changes/updates:

1. update to most recent Python versions for testing
2. remove Python 2 __future__ module imports
3. move intermediate Python versions to weekly testing and only test PRs with oldest and newest versions
4. update caches since I changed the gold standard to yt-3.6.0
5. remove testing entries for Trident master branch. It was unnecessary of me to include those in this branch.

[coveralls]

Pull Request Test Coverage Report for Build 639

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 34 unchanged lines in 1 file lost coverage.
• Overall coverage decreased (-0.01%) to 75.407%

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Files with Coverage Reduction	New Missed Lines	%
/home/circleci/trident/trident/absorption_spectrum/absorption_spectrum.py	34	92.37%

Totals
Change from base Build 498:	-0.01%
Covered Lines:	1852
Relevant Lines:	2456

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💛 - Coveralls

[chummels]

Thanks for doing this! Looking forward to just having master soon!