support x509 request challenge password parsing (#4944)

* support x509 request challenge password parsing

* switch to a more generic (but not too generic) attribute parsing

* make it raise a valueerror

* Update tests/x509/test_x509.py

Co-authored-by: Alex Gaynor <alex.gaynor@gmail.com>

Co-authored-by: Alex Gaynor <alex.gaynor@gmail.com>