full set of parameters that rdata's from_text() allows (i.e. origin,
relativize, and relativize_to).

These parameters were added though the order is slightly different
from dns.rdata.to_text() as in the the rdata version idna_codec is
after origin, relativize, and relativize_to.  We already had an
idna_codec for the Rdataset and RRset functions, so we added after it
to keep backwards compatibility.

Classes inherit from object by default; there's no need to explicitly
include this.

Replace super(Foo, self) with super().

These methods (which convert a str/int into an enum/int) shouldn't be
commonly used by external code, so don't need to exist at the module
level.  The make() method on the enum class (renamed from to_enum()) can
still be used, and the internal callers have been updated to use it.

Previously, repr() of rdataset/rrset looked like this:
<DNS IN A rdataset>
<DNS IN TXT rdataset>
<DNS IN RRSIG rdataset>

<DNS example. IN A RRset>
<DNS example. IN TXT RRset>
<DNS example. IN RRSIG(NSEC) RRset>

With this patch, it they look like:
<DNS IN A rdataset: [<1.2.3.4>, <5.6.7.8>]>
<DNS IN TXT rdataset: [<"foo" "bar">, <"baz">]>
<DNS IN RRSIG(NSEC) rdataset: [<NSEC 1 3 3600 20200101000000 20030101000000 2143 foo MxFcby9k/yvedMfQgKzhH5er0Mu/vILz 45IkskceFGgiWC...>]>

<DNS example. IN A RRset: [<1.2.3.4>, <5.6.7.8>]>
<DNS example. IN TXT RRset: [<"foo" "bar">, <"baz">]>
<DNS example. IN RRSIG(NSEC) RRset: [<NSEC 1 3 3600 20200101000000 20030101000000 2143 foo MxFcby9k/yvedMfQgKzhH5er0Mu/vILz 45IkskceFGgiWC...>]>

Note that each rdata is truncated to 100 characters.