Modernize setuptools usage in pyproject.toml

1. Remove the redundant `wheel` dependency.  The setuptools build
   backend has been adding it automatically since day one, and it was
   explicitly specified in the docs as a mistake.  See:
   https://github.com/pypa/setuptools/commit/f7d30a9529378cf69054b5176249e5457aaf640a

2. Replace the legacy backend with the regular backend.  The legacy
   backend was only intended to be used implicitly when `pyproject.toml`
   does not specify only, and was not supposed to be specified
   explicitly there.  See:
   https://github.com/pypa/setuptools/issues/1689

3. Prepend the current directory to `sys.path` as required for
   `setup.py` to reliably import `setupbase`.  The non-legacy backend
   no longer does this for us.

pyproject.toml

@@ -1,3 +1,3 b''
 [build-system]
-requires = ["setuptools >= 51.0.0", "wheel"]
-build-backend = "setuptools.build_meta:__legacy__"
+requires = ["setuptools >= 51.0.0"]
+build-backend = "setuptools.build_meta"

setup.py

@@ -64,6 +64,8 b' Python {py} detected.'
 from setuptools import setup
 
 # Our own imports
+sys.path.insert(0, ".")
+
 from setupbase import target_update
 
 from setupbase import (