fix code_verifier length

[mamico]

In the current implementation, code_verifier can exceed the length limits defined in the specification (43-128) because, once the byte string of the correct length is defined, it is base64 encoded to comply with the accepted character constraint, which increases its length.

An alternative implementation, as in: https://github.com/RomeoDespres/pkce/blob/master/pkce/__init__.py#L19 could be

code_verifier = secrets.token_urlsafe(96)[:length]

[peppelinux]

ILGTM

I believe that also the code snippet contained in the main documentation should be aligned to this PR https://github.com/italia/spid-cie-oidc-docs/blob/a65aa1a6ccc2bf03d8c18dd8df2ee22ec2613316/static/pkce.py#L6