Fixes urllib3#1683.

Recently merged urllib3#1673 ensured that URLs are percent-encoded.
This is a behavior change in that URLs with tilde characters in
them would not previously have been percent-encoded, but are now.

RFC 3986 says[1]:

   For consistency, percent-encoded octets in the ranges of ALPHA
   (%41-%5A and %61-%7A), DIGIT (%30-%39), hyphen (%2D), period (%2E),
   underscore (%5F), or tilde (%7E) should not be created by URI
   producers and, when found in a URI, should be decoded to their
   corresponding unreserved characters by URI normalizers.

This suggests that urllib3 should not escape tilde characters
in URLs.  The RFC describes tilde as an "unreserved" character.
Among the character classes at the top of url.py, the closest
match to the "unreserved" set seems to be ZONE_ID_CHARS.  RFC6874
says:

    A <zone_id> SHOULD contain only ASCII characters classified as
    "unreserved" for use in URIs [RFC3986].

Which suggests that it should be safe to add to that set.

[1] https://tools.ietf.org/html/rfc3986#section-2.3
[2] https://tools.ietf.org/html/rfc6874#section-2