Upgrade syntax for Python 3.10+ and remove redundant Python 2 code

[hugovk]

Description

Upgrade the syntax with pyupgrade and remove an old Python 2 block.

PR Checklist:

• Tests that fully cover new/changed functionality.
• [n/a] Documentation including tutorial content if appropriate.
• [n/a] Changelogs, if there are API changes.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.76%. Comparing base (6b9fb79) to head (c04de15).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3372      +/-   ##
==========================================
- Coverage   89.76%   89.76%   -0.01%     
==========================================
  Files          29       29              
  Lines       31292    31286       -6     
  Branches     5738     5738              
==========================================
- Hits        28089    28083       -6     
  Misses       1794     1794              
  Partials     1409     1409              

Flag	Coverage Δ
c-tests	86.77% <ø> (ø)
lwt-tests	80.38% <ø> (ø)
python-c-tests	87.12% <ø> (ø)
python-tests	98.84% <100.00%> (-0.01%)	⬇️
python-tests-no-jit	33.46% <100.00%> (-0.05%)	⬇️
python-tests-numpy1	50.24% <100.00%> (-0.04%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
python/tskit/drawing.py	98.39% <100.00%> (-0.01%)	⬇️
python/tskit/metadata.py	98.89% <100.00%> (ø)
python/tskit/tables.py	99.77% <100.00%> (-0.01%)	⬇️
python/tskit/util.py	99.27% <100.00%> (-0.01%)	⬇️

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[jeromekelleher]

LGTM, thanks!