15110: set env var for subprocess, check version in script (#15122)

Fixes #15110 

- Add PYTHONSAFEPATH="true" to call to subprocess in get_search_dirs()
- Check sys.version_info before modifying path in pyinfo.py 

To test, import mypy.modulefinder.get_search_dirs & run:
- get_search_dirs() with no args
- on Python 3.11, call get_search_dirs with an earlier version as
python_executable
- on Python < 3.11, call get_search_dirs with Python 3.11 as
python_executable

Expected:
- consistent results, no exceptions

Co-authored-by: eevel <eevel@weezel3.weezelnet>
Co-authored-by: Shantanu <12621235+hauntsaninja@users.noreply.github.com>
Co-authored-by: Alex Waygood <Alex.Waygood@Gmail.com>

mypy/modulefinder.py

@@ -751,12 +751,19 @@ def get_search_dirs(python_executable: str | None) -> tuple[list[str], list[str]
     else:
         # Use subprocess to get the package directory of given Python
         # executable
+        env = {**dict(os.environ), "PYTHONSAFEPATH": "1"}
         try:
             sys_path, site_packages = ast.literal_eval(
                 subprocess.check_output(
-                    [python_executable, pyinfo.__file__, "getsearchdirs"], stderr=subprocess.PIPE
+                    [python_executable, pyinfo.__file__, "getsearchdirs"],
+                    env=env,
+                    stderr=subprocess.PIPE,
                 ).decode()
             )
+        except subprocess.CalledProcessError as err:
+            print(err.stderr)
+            print(err.stdout)
+            raise
         except OSError as err:
             reason = os.strerror(err.errno)
             raise CompileError(

mypy/pyinfo.py

@@ -12,12 +12,13 @@
 if __name__ == "__main__":
     # HACK: We don't want to pick up mypy.types as the top-level types
     #       module. This could happen if this file is run as a script.
-    #       This workaround fixes it.
-    old_sys_path = sys.path
-    sys.path = sys.path[1:]
-    import types  # noqa: F401
+    #       This workaround fixes this for Python versions before 3.11.
+    if sys.version_info < (3, 11):
+        old_sys_path = sys.path
+        sys.path = sys.path[1:]
+        import types  # noqa: F401
 
-    sys.path = old_sys_path
+        sys.path = old_sys_path
 
 import os
 import site