TST: install pip 24.1b1 or later when on Python 3.13t

Earlier pip releases do not recognize the free-threading ABI flag.

tests/conftest.py

@@ -10,6 +10,7 @@
 import shutil
 import subprocess
 import sys
+import sysconfig
 import tempfile
 import warnings
 
@@ -77,13 +78,18 @@ def tmp_path_session(tmp_path_factory):
 class VEnv(EnvBuilder):
  def __init__(self, env_dir):
  super().__init__(symlinks=True, with_pip=True)
+
  # This warning is mistakenly generated by CPython 3.11.0
  # https:/python/cpython/pull/98743
  with warnings.catch_warnings():
  if sys.version_info[:3] == (3, 11, 0):
  warnings.filterwarnings('ignore', 'check_home argument is deprecated and ignored.', DeprecationWarning)
  self.create(env_dir)
 
+ # Free-threaded Python 3.13 requires pip 24.1b1 or later.
+ if sysconfig.get_config_var('Py_GIL_DISABLED'):
+ self.pip('install', '--upgrade', 'pip >= 24.1b1')
+
  def ensure_directories(self, env_dir):
  context = super().ensure_directories(env_dir)
  # Store the path to the venv Python interpreter. There does