Why can't URL.build() accept a non-empty scheme with an empty host?

[atemate]

Why does URL.build has this check:

yarl/yarl/__init__.py

Lines 201 to 202 in 5ef628f

	if not host and scheme:
	raise ValueError('Can\'t build URL with "scheme" but without "host".')

From my perspective, URL with a scheme and without a host is valid:

In [12]: u = URL("scheme://")                                                                                                                                                                 

In [13]: u.scheme, u.host                                                                                                                                                                     
Out[13]: ('scheme', None)

In [14]: URL.build(scheme=u.scheme, host=u.host)                                                                                                                                              
---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
<ipython-input-14-c54f9b7a765d> in <module>
----> 1 URL.build(scheme=u.scheme, host=u.host)
...
    200 
    201         if not host and scheme:
--> 202             raise ValueError('Can\'t build URL with "scheme" but without "host".')
    203         if port and not host:
    204             raise ValueError('Can\'t build URL with "port" but without "host".')

ValueError: Can't build URL with "scheme" but without "host".

[asvetlov]

Oversight, probably.
Plus, relaxing the rule is much easier from backward-compatibility point of view than the restricting.