Allow scheme replacement for relative urls

[asvetlov]

URL('path/to').with_scheme('http') should work.
Now it produces scheme replacement is not allowed for relative URLs

[aio-libs-bot]

GitMate.io thinks possibly related issues are #185 (Allow joining URL and pathlib.Path), #99 (Document-relative urls like '?a=b'), #33 (Parse URL parameter), #272 (bool(URL()) should be False), and #24 (TypeError in URL.with_query).

[mjpieters]

File URLs that are absolute are also being blocked from updating the scheme:

>>> URL('file:///absolute/path').with_scheme('file')
Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File "/.../lib/python3.7/site-packages/yarl/_url.py()", line 794, in with_scheme
    raise ValueError("scheme replacement is not allowed for relative URLs")
ValueError: scheme replacement is not allowed for relative URLs

This is rather a confusing error.

[bdraco]

The http case is correct because the host is not allowed to be undefined for host see #821

The file case isn't correct though