Update PSS verify signature code example.

[jelaiw]

For completeness sake, update code example for verifying a signature (#783 just updated the signing code example).

Patch does the following:

• Update RSA.import_key() to read DER file in binary.
• Remove TypeError from except clause.
  • Does not appear that verify() should throw this exception; or that the caller is expected to do anything based on API docs at https://www.pycryptodome.org/src/signature/pkcs1_pss#Crypto.Signature.pss.PSS_SigScheme.verify.
  • Also, checked the unit tests, the "negative" tests are just checking for ValueError.
• Update print statements for Python 3.

make html appears to build the docs as expected.

Hope this helps!

[Legrandin]

Thanks!