feat: support pypa build (#521)

* feat: support pypa build

fix: update `cibuildwheel.linux.troubleshoot` to know about build

Workaround issue with PyPy venv module by installing build[virtualenv]

fix: test_dependency_constraints_file when using build

fix: test/test_pep518.py

fix: use `strtobool` to parse `CIBW_PYPA_BUILD`

fix: update `cibuildwheel.linux.troubleshoot` to know about `python -m pip wheel`

test: use `build_mode` in `test/test_dependency_versions.py` tests

* refactor: use build-backend instead

* Apply suggestions from code review

Co-authored-by: Joe Rickerby <joerick@mac.com>

* refactor: use literal types for build_frontend

* refactor(tests): build_mode -> build_frontend_env

* Update test/test_before_build.py

Co-authored-by: Joe Rickerby <joerick@mac.com>

test/test_troubleshooting.py

@@ -16,7 +16,7 @@ raise Exception('this build will fail')
 """
 
 
-def test_failed_project_with_so_files(tmp_path, capfd):
+def test_failed_project_with_so_files(tmp_path, capfd, build_frontend_env):
     if utils.platform != "linux":
         pytest.skip("this test is only relevant to the linux build")
 
@@ -24,7 +24,7 @@ def test_failed_project_with_so_files(tmp_path, capfd):
     so_file_project.generate(project_dir)
 
     with pytest.raises(subprocess.CalledProcessError):
-        utils.cibuildwheel_run(project_dir)
+        utils.cibuildwheel_run(project_dir, add_env=build_frontend_env)
 
     captured = capfd.readouterr()
     print("out", captured.out)