Fix using setuptools.build_meta backend

Add the current directory path to `sys.path` explicitly, in order to fix
using the correct `setuptools.build_meta` backend.  The legacy backend
is only meant to be used implicitly for packages that have not been
ported to PEP 517 builds yet, and using it explicitly is invalid.
The correct solution here is to set the import path correctly.

pyproject.toml

@@ -76,9 +76,7 @@ requires = [
     "requests >= 2.26.0",
     "sip == 6.10.0",
 ]
-# Using "setuptools.build_meta:__legacy__" instead of "setuptools.build_meta" for now.
-# Allows to have access to the folder on the search path when building, like before.
-build-backend = "setuptools.build_meta:__legacy__"
+build-backend = "setuptools.build_meta"
 
 [tool.setuptools.packages.find]
 exclude = ["src", "buildtools*", "etgtools", "sphinxtools", "src", "unittests"]

setup.py

@@ -20,6 +20,9 @@ from setuptools.command.bdist_egg   import bdist_egg as orig_bdist_egg
 from setuptools.command.sdist       import sdist as orig_sdist
 from setuptools.command.bdist_wheel import bdist_wheel as orig_bdist_wheel
 
+# Alter the path so that buildtools can be imported from the current directory.
+sys.path.insert(0, os.path.dirname(__file__))
+
 from buildtools.config import Config, msg, opj, runcmd, canGetSOName, getSOName
 import buildtools.version as version