Update PSS docs for Python 3.

[jelaiw]

While trying the code example (for signing) at https://www.pycryptodome.org/src/signature/pkcs1_pss, I ran into a few exceptions (in Python 3.10). Figured this may be a good time to update this example.

Patch does the following:

• Update message to byte string (otherwise SHA256.new() throws an exception).
• Update RSA.import_key() to read DER file in binary (otherwise this call throws an exception too; plus, pss.new() expects a byte string now).
• Removed an unused import statement for Crypto.Random.

make html appears to build the docs as expected.

Hope this helps!

[Legrandin]

Thanks!