Roadmap to rez 3.0

[JeanChristopheMorinPerso]

This issue tracks the roadmap I have in mind for a 3.0 release of rez. I don't think anythng that I included will be or is controversial. I tried to keep it as conservative as possible.

2.114.0 (last release in the 2.x series)

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1. Continue on missing variant packages #1550
   enhancement +1
2. Drop support for installs with Python lower than 3.7 #1469
   +0
3. Rename rez.vendor.version to rez.version and define a public API for rez.version #1534
   +0
4. Deprecated rez 1 compatibility settings so that we can remove them in the future
5. Mark anything in Remove things that are officially marked as deprecated #1552 that is not yet marked as deprecated, as deprecated.

(see the 2.114.0 milestone)

3.0.0

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1. Drop support for Python 2 entirely #1551
   enhancement +1
2. Remove rez1 compatibility settings #1553
   enhancement +1
3. Remove bez #1561
   +0
4. Remove rez.vendor.version
5. Windows Gitbash Support Fixes #1302 #1475
   os:windows run-benchmarks shell +3
6. Make rez wheels production ready #1536
   installer +1

(see the 3.0.0 milestone)