Fix get_preferred_scheme detection for CPython 3.10 alpha

Co-authored-by: Sylvain Desodt <sylvain.desodt@faurecia.com>
Co-authored-by: Tzu-ping Chung <uranusjr@gmail.com>

news/10252.bugfix.rst

@@ -0,0 +1,2 @@
+Modify the ``sysconfig.get_preferred_scheme`` function check to be
+compatible with CPython 3.10’s alphareleases.

src/pip/_internal/locations/_sysconfig.py

@@ -24,7 +24,7 @@
 
 _AVAILABLE_SCHEMES = set(sysconfig.get_scheme_names())
 
-_HAS_PREFERRED_SCHEME_API = sys.version_info >= (3, 10)
+_PREFERRED_SCHEME_API = getattr(sysconfig, "get_preferred_scheme", None)
 
 
 def _infer_prefix() -> str:
@@ -41,8 +41,8 @@ def _infer_prefix() -> str:
 
     If none of the above works, fall back to ``posix_prefix``.
     """
-    if _HAS_PREFERRED_SCHEME_API:
-        return sysconfig.get_preferred_scheme("prefix")  # type: ignore
+    if _PREFERRED_SCHEME_API:
+        return _PREFERRED_SCHEME_API("prefix")
     os_framework_global = is_osx_framework() and not running_under_virtualenv()
     if os_framework_global and "osx_framework_library" in _AVAILABLE_SCHEMES:
         return "osx_framework_library"
@@ -61,8 +61,8 @@ def _infer_prefix() -> str:
 
 def _infer_user() -> str:
     """Try to find a user scheme for the current platform."""
-    if _HAS_PREFERRED_SCHEME_API:
-        return sysconfig.get_preferred_scheme("user")  # type: ignore
+    if _PREFERRED_SCHEME_API:
+        return _PREFERRED_SCHEME_API("user")
     if is_osx_framework() and not running_under_virtualenv():
         suffixed = "osx_framework_user"
     else:
@@ -76,8 +76,8 @@ def _infer_user() -> str:
 
 def _infer_home() -> str:
     """Try to find a home for the current platform."""
-    if _HAS_PREFERRED_SCHEME_API:
-        return sysconfig.get_preferred_scheme("home")  # type: ignore
+    if _PREFERRED_SCHEME_API:
+        return _PREFERRED_SCHEME_API("home")
     suffixed = f"{os.name}_home"
     if suffixed in _AVAILABLE_SCHEMES:
         return suffixed