Fix Python bindings of `X509Certificate`

[touilleMan]

See for instance this seemingly correct code:

    try:
        certificates = [
            X509Certificate.from_der(raw_leaf),
            *(X509Certificate.from_der(raw) for raw in raw_intermediates)
        ]
    except ValueError:
        return ValidateX509TrustchainBadOutcome.INVALID_DER_X509_CERTIFICATE

    current = certificates[0]
    already_seen: set[bytes] = set()
    while True:
        already_seen.add(current.subject())
...

The trick is X509Certificate.from_der never raise any exception ! Instead the actual certificate validation is done lazily when calling current.subject() :/

This is error prone and not needed. Instead we should:

• Have X509Certificate.from_der doing the validation and raise a ValueError if the provided certificate is invalid
• Rename X509Certificate.try_from_pem into X509Certificate.from_pem and also have this function do the validation eagerly (the try_ prefix is not something Pythonic)
• Replace the subject() and issuer() method by simple properties