Version 2.5.1

Bugfix release.

Minor Improvements and bugfixes

• Use a fixed datetime to avoid test failures (#520) (@narendravardi)
• Parse multi-line future imports better (#519) (@akx)
• Fix validate_currency docstring (#522)
• Allow normalize_locale and exists to handle various unexpected inputs (#523) (@suhojm)
• Make PyInstaller support more robust (#525, #526) (@thijstriemstra, @akx)