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>

cibuildwheel/util.py

@@ -4,6 +4,8 @@ import itertools
 import os
 import re
 import ssl
+import subprocess
+import sys
 import textwrap
 import time
 import urllib.request
@@ -20,12 +22,14 @@ from packaging.version import Version
 
 from .architecture import Architecture
 from .environment import ParsedEnvironment
-from .typing import PathOrStr, PlatformName
+from .typing import Literal, PathOrStr, PlatformName
 
 resources_dir = Path(__file__).parent / "resources"
 
 install_certifi_script = resources_dir / "install_certifi.py"
 
+BuildFrontend = Literal["pip", "build"]
+
 
 def prepare_command(command: str, **kwargs: PathOrStr) -> str:
     """
@@ -218,6 +222,7 @@ class BuildOptions(NamedTuple):
     test_requires: List[str]
     test_extras: str
     build_verbosity: int
+    build_frontend: BuildFrontend
 
 
 class NonPlatformWheelError(Exception):
@@ -300,3 +305,18 @@ def print_new_wheels(msg: str, output_dir: Path) -> Iterator[None]:
     s = time.time() - start_time
     m = s / 60
     print(msg.format(n=n, s=s, m=m), *sorted(f"  {f.name}" for f in new_contents), sep="\n")
+
+
+def get_pip_version(env: Dict[str, str]) -> str:
+    # we use shell=True here for windows, even though we don't need a shell due to a bug
+    # https://bugs.python.org/issue8557
+    shell = sys.platform.startswith("win")
+    versions_output_text = subprocess.check_output(
+        ["python", "-m", "pip", "freeze", "--all"], universal_newlines=True, shell=shell, env=env
+    )
+    (pip_version,) = (
+        version[5:]
+        for version in versions_output_text.strip().splitlines()
+        if version.startswith("pip==")
+    )
+    return pip_version