build!: drop Python 2 support, require Python 3.6+

Summary

Drop Python 2 support entirely. Python 2.7 reached end-of-life on January 1, 2020.

• Remove all from __future__ imports
• Remove if str is bytes: version checks
• Remove Python 2 CI job
• Simplify pylib/poly.py compatibility layer
• Update build to require Python >= 3.6.0

Changes

Area	Before	After
pylib/poly.py	114 lines	71 lines
pylib/util.py termsize()	30+ lines	6 lines
CI jobs	Includes Python 2	Python 3 only

Net: -182 lines of code

Test plan

• waf configure shows "python version >= 3.6.0"
• waf build passes
• waf check passes (all tests)
• CI pipeline

Closes discussion in !1506 where James suggested killing Python 2 entirely.