TST: remove pep518 tests

These tests verify that build can build sdists and wheels using
meson-python. The pep518 name is misleading as meson-python itself
does not implement PEP 518. These tests verify that the PEP 517 hooks
exported by meson-python work as build expects them to work.

These tests setup a local directory with wheel for the build
dependencies. This is problematic for two reasons: it requires network
access, thus distributions need to disable these tests with downstream
patches, and it makes it very difficult to run the tests against a
specific version of meson.

Creation of wheels and sdists is tested as part of the unit tests. The
pep517 tests are unit tests for some of the PEP 517 hooks. The
installation of meson-python itself in the CI jobs works as
integration tests.

Remove these tests.

Author: dnicolodi

tests/conftest.py

@@ -152,25 +152,3 @@ def disable_pip_version_check():
     mpatch = pytest.MonkeyPatch()
     yield mpatch.setenv('PIP_DISABLE_PIP_VERSION_CHECK', '1')
     mpatch.undo()
-
-
-@pytest.fixture(scope='session')
-def pep518_wheelhouse(tmp_path_factory):
-    wheelhouse = os.fspath(tmp_path_factory.mktemp('wheelhouse'))
-    meson_python = os.fspath(package_dir.parent.parent)
-    # Populate wheelhouse with wheel for the following packages and
-    # their dependencies.  Wheels are downloaded from PyPI or built
-    # from the source distribution as needed.  Sources or wheels in
-    # the pip cache are used when available.
-    packages = [
-        meson_python,
-    ]
-    cmd = [sys.executable, '-m', 'pip', 'wheel', '--no-build-isolation', '--wheel-dir', wheelhouse, *packages]
-    subprocess.run(cmd, check=True)
-    return wheelhouse
-
-
-@pytest.fixture
-def pep518(pep518_wheelhouse, monkeypatch):
-    monkeypatch.setenv('PIP_FIND_LINKS', pep518_wheelhouse)
-    monkeypatch.setenv('PIP_NO_INDEX', 'true')