fix X509Extension __str__() method for unknow extension types

[ich199]

fixes: #1238
fixes: #270

Unsure if it would be preferred to use 65536 instead of 1 << 16 when defining X509V3_EXT_ERROR_UNKNOWN, but I copied the syntax used in OpenSSL

[ich199]

tests seem to be failing due to #1240

[ich199]

tests are now failing due to an issue in a new release of sphinx (7.2.5) sphinx-doc/sphinx#11662

[mhils]

Thanks @ich199! The change generally makes sense to me. Would you mind adding a X509V3_EXT_ERROR_UNKNOWN binding to cryptography and then use this here instead of defining a custom constant?

[ich199]

Thanks @mhils, I've created the binding in cryptography as requested and that's been merged. I'm guessing it would be preferred to backport that to 41.0.x rather than bumping the cryptography requirement here?

[mhils]

Awesome, thanks! Bumping the cryptography requirement here is perfectly fine (and preferred), cryptography and pyOpenSSL releases go more or less in tandem. :)