Support URL.build() with scheme and path (creates a relative URL)

[asvetlov]

Fixes #464

[codecov]

Codecov Report

Merging #479 into master will decrease coverage by 0.00%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #479      +/-   ##
==========================================
- Coverage   99.57%   99.56%   -0.01%     
==========================================
  Files           2        2              
  Lines         699      697       -2     
  Branches      157      156       -1     
==========================================
- Hits          696      694       -2     
  Misses          3        3              

Impacted Files	Coverage Δ
yarl/__init__.py	99.43% <ø> (-0.01%)	⬇️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 30a6eb9...e4063c9. Read the comment docs.