Declare setuptools as a build backend.

Confirmed this change has the intended effect:
draft $ pip-run git+https://github.com/jaraco/genshi@bugfix/64-declare-build-backend -- -c pass
Collecting git+https://github.com/jaraco/genshi@bugfix/64-declare-build-backend
  Cloning https://github.com/jaraco/genshi (to revision bugfix/64-declare-build-backend) to /private/var/folders/c6/v7hnmq453xb6p2dbz1gqc6rr0000gn/T/pip-req-build-l1t6wu4r
  Running command git clone --filter=blob:none --quiet https://github.com/jaraco/genshi /private/var/folders/c6/v7hnmq453xb6p2dbz1gqc6rr0000gn/T/pip-req-build-l1t6wu4r
  Running command git checkout -b bugfix/64-declare-build-backend --track origin/bugfix/64-declare-build-backend
  Switched to a new branch 'bugfix/64-declare-build-backend'
  branch 'bugfix/64-declare-build-backend' set up to track 'origin/bugfix/64-declare-build-backend'.
  Resolved https://github.com/jaraco/genshi to commit 245ac522bf3f5ac298036d405d66fedca185dc3b
  Installing build dependencies ... done
  Getting requirements to build wheel ... done
  Installing backend dependencies ... done
  Preparing metadata (pyproject.toml) ... done
Collecting six
  Using cached six-1.16.0-py2.py3-none-any.whl (11 kB)
Building wheels for collected packages: Genshi
  Building wheel for Genshi (pyproject.toml) ... done
  Created wheel for Genshi: filename=Genshi-0.8-cp310-cp310-macosx_10_9_universal2.whl size=189717 sha256=b1dcfb4f310c61683c0b797b7bc60811409ce8555d9c0986bf7016940698cec3
  Stored in directory: /private/var/folders/c6/v7hnmq453xb6p2dbz1gqc6rr0000gn/T/pip-ephem-wheel-cache-0t3dbq_e/wheels/ec/93/70/3b03c99961bc623b8d7d3d514c0faacedb07db6d565bfbba2b
Successfully built Genshi
Installing collected packages: six, Genshi
Successfully installed Genshi-0.8 six-1.16.0
edgewall / genshi

Declare setuptools as a build backend. #65
