Vendorize keyring

This PR vendorizes (aka bundle) both keyring and win32ctypes so that enstaller always use the same authentication mechanism, independantly of what's installed (keyring version, pywin32, etc...).

Beyond bundling keyring (3.3), we did the following:

  - we transformed the absolute imports into explicit relative imports to make bundling possible without dirty sys.path hacks
  - we used the bundled win32-ctypes instead of pywin32