Backwards incompatible change to DNSName (#3951)

* Backwards incompatible change to DNSName

During this release cycle we decided to officially deprecate passing
U-labels to our GeneralName constructors. At first we tried changing
this in a purely backwards compatible way but get_values_for_type made
that untenable. This PR modifies DNSName to take three different types.
U-label strings (which raises a deprecation warning), A-label strings
(the new preferred type), and bytes (which are assumed to be decodable
to unicode strings). The latter, while supported, is primarily intended
for use by our parser and allows us to return the actual encoded data in
a certificate even if it has not been properly encoded to A-label before
the certificate is created. (Of course, if the certificate contains
invalid utf8 sequences this will still fail, but let's handle one
catastrophic failure at a time).

* coverage

* don't delete that asterisk from a test. it does things.

* no bytes in DNSName. Private constructor for bypassing validation

* test unicode in dnsname (yuck)

* fix docs

* empty commit, you disappoint me codecov

* CI is the worst

1 files changed, 1 insertions, 1 deletions

diff --git a/docs/x509/reference.rst b/docs/x509/reference.rst
index 951e6b7d..238a24e5 100644
--- a/docs/x509/reference.rst
+++ b/docs/x509/reference.rst
@@ -611,7 +611,7 @@ X.509 Certificate Builder
         >>> builder = builder.public_key(public_key)
         >>> builder = builder.add_extension(
         ...     x509.SubjectAlternativeName(
-        ...         [x509.DNSName(b'cryptography.io')]
+        ...         [x509.DNSName(u'cryptography.io')]
         ...     ),
         ...     critical=False
         ... )