Allow to decide whether to use distutils or sysconfig with sysconfig.…

…_PIP_USE_SYSCONFIG

Fixes pypa#10647

src/pip/_internal/locations/__init__.py

@@ -45,7 +45,20 @@
 
 _PLATLIBDIR: str = getattr(sys, "platlibdir", "lib")
 
-_USE_SYSCONFIG = sys.version_info >= (3, 10)
+
+def _should_use_sysconfig() -> bool:
+    """
+    This function determines the value of _USE_SYSCONFIG.
+    By default, pip uses sysconfig on Python 3.10+.
+    But Python distributors can override this decision by setting:
+        sysconfig._PIP_USE_SYSCONFIG = True / False
+    Rationale in https://github.com/pypa/pip/issues/10647
+    """
+    return bool(getattr(sysconfig, "_PIP_USE_SYSCONFIG", sys.version_info >= (3, 10)))
+
+
+# This is a function for testability, but should be constant during any one run.
+_USE_SYSCONFIG = _should_use_sysconfig()
 
 
 def _looks_like_bpo_44860() -> bool: