pth files are evil.  They are very difficult to debug because they're processed too early.  They usually contain globs of inscrutable code.  Exceptions in pth files can get swallowed in some cases.  They are loaded in indeterminate order.

They are also unnecessary to support namespace packages in Python 3 (ignoring straddling code).

Let's start the process for removing them.

1. Deprecate pth files in Python 3.8 and turn them off with the -3 option.

2. Kill off pth file support once Python 2 is EOL'd.