fix(domain): accept single-label TLDs erroneously rejected by domain() and hostname()

[eeshsaxena]

Fixes #442.

domain() and hostname() with
fc_1034=True rejected valid single-label domain names (ie, com, yu, ie.) because the internal regex used a + quantifier on the label group — which forces at least one dot separator. So a bare TLD like ie never matched.

Multi-label names like defo.ie and yugoslavia.yu were fine; only the single-label case was broken.

RFC 1034 §2.3.1 explicitly permits single-label names. The fix adds a second match path after the existing multi-label path: a bare TLD must be 2-63 alpha characters (optionally followed by a trailing dot when
fc_1034=True). Pure-digit strings like 123 are still rejected, consistent with the multi-label path.

Before:
\\python
domain('ie') # ValidationError (wrong)
domain('ie.') # ValidationError (wrong)
domain('com') # ValidationError (wrong)
hostname('ie.', rfc_1034=True) # ValidationError (wrong)
\\

After:
\\python
domain('ie') # True
domain('ie.', rfc_1034=True) # True
domain('ie.') # ValidationError (correct — trailing dot only allowed with rfc_1034)
domain('com') # True
hostname('ie.', rfc_1034=True) # True
domain('123') # ValidationError (still rejected)
\\

Multi-label domains, IDN, and all existing behaviour unchanged.