bpo-38927: Use python -m pip to upgrade venv deps (GH-17403)

I suggest you add `bpo-NNNNN: ` as a prefix for the first commit for future PRs. Thanks!

Lib/test/test_venv.py

@@ -141,16 +141,18 @@ class BasicTest(BaseTest):
     def test_upgrade_dependencies(self):
         builder = venv.EnvBuilder()
         bin_path = 'Scripts' if sys.platform == 'win32' else 'bin'
-        pip_exe = 'pip.exe' if sys.platform == 'win32' else 'pip'
+        python_exe = 'python.exe' if sys.platform == 'win32' else 'python'
         with tempfile.TemporaryDirectory() as fake_env_dir:
 
             def pip_cmd_checker(cmd):
                 self.assertEqual(
                     cmd,
                     [
-                        os.path.join(fake_env_dir, bin_path, pip_exe),
+                        os.path.join(fake_env_dir, bin_path, python_exe),
+                        '-m',
+                        'pip',
                         'install',
-                        '-U',
+                        '--upgrade',
                         'pip',
                         'setuptools'
                     ]

Lib/venv/__init__.py

@@ -393,10 +393,10 @@ class EnvBuilder:
             f'Upgrading {CORE_VENV_DEPS} packages in {context.bin_path}'
         )
         if sys.platform == 'win32':
-            pip_exe = os.path.join(context.bin_path, 'pip.exe')
+            python_exe = os.path.join(context.bin_path, 'python.exe')
         else:
-            pip_exe = os.path.join(context.bin_path, 'pip')
+            python_exe = os.path.join(context.bin_path, 'python')
-        cmd = [pip_exe, 'install', '-U']
+        cmd = [python_exe, '-m', 'pip', 'install', '--upgrade']
         cmd.extend(CORE_VENV_DEPS)
         subprocess.check_call(cmd)
 

Misc/NEWS.d/next/Library/2019-11-27-17-47-00.bpo-38927.qT7xKY.rst

@@ -0,0 +1 @@
+Use ``python -m pip`` instead of ``pip`` to upgrade dependencies in venv.