Test code and version support update

[jtisbell4]

• Reconfigured tox.ini
• Moved DBR-specific requirements into python/tests/requirements
• Made SparkSession initialization dynamic in test class setup
• Fixed if statement in TSDF.withRangeStats
• Removed event_time column assumption and made Z-ORDER attempt contingent on optimizationCols in io.write

[CLAassistant]

All committers have signed the CLA.

[R7L208]

I'd like to agree on if we should be testing for Delta versions that support OPTIMIZE and Z-ORDER, as well as whether or not to use DBR for tox env names, before merging but otherwise LGTM

[tnixon]

I'm seeing a lot of changes to the docs and other parts of the code not directly related to Tox and the test code. I suspect these might be from work-in-progress changes from another branch that got merged into your working branch @jtisbell4? In any event, let's clean this up so we're only making changes related to this particular update.

[jtisbell4]

I'm seeing a lot of changes to the docs and other parts of the code not directly related to Tox and the test code. I suspect these might be from work-in-progress changes from another branch that got merged into your working branch @jtisbell4? In any event, let's clean this up so we're only making changes related to this particular update.

Apologies. Fixed.

[tnixon]

Need a few changes to io.py - see inline comments

[tnixon]

LGTM