Use Sequence instead of List for nameservers, as List is invariant

[av223119]

without this, resolver.nameservers = string.split() produces mypy error, see
https://mypy.readthedocs.io/en/stable/common_issues.html#invariance-vs-covariance

[av223119]

mypy error message:

error: Incompatible types in assignment (expression has type "list[str]", variable has type "list[Union[str, Nameserver]]")  [assignment]
note: "List" is invariant -- see https://mypy.readthedocs.io/en/stable/common_issues.html#variance
note: Consider using "Sequence" instead, which is covariant

code in question

my_resolver = dns.resolver.Resolver()
my_resolver.nameservers = os.environ["DNS"].split()

[rthalley]

Thanks!