windows: upgrade pip with 'python -m pip'

this fixes an "ImportError: cannot import name main" when self-upgrading
pip to latest version 10

see https://github.com/pypa/pip/issues/5240#issuecomment-382989420

cibuildwheel/windows.py

@@ -72,7 +72,7 @@ def build(project_dir, package_name, output_dir, test_command, test_requires, be
         shell(['python', '-c', '"import struct; print(struct.calcsize(\'P\') * 8)\"'], env=env)
 
         # prepare the Python environment
-        shell(['pip', 'install', '--disable-pip-version-check', '--user', '--upgrade', 'pip'],
+        shell(['python', '-m', 'pip', 'install', '--upgrade', 'pip'],
               env=env)
         shell(['pip', 'install', 'wheel'], env=env)