yarl: URL.build doesn't url encode credentials

EDIT the bug described here is just a consequence of the bug described below.

Currently square brackets in the username or the password lead to the URL being interpreted as an IPv6 address:

>>> yarl.URL("https://x:[@localhost")
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/Users/hynek/.virtualenvs/ansible-tools/lib/python3.6/site-packages/yarl/__init__.py", line 149, in __init__
    val = urlsplit(val)
  File "/Users/hynek/.pyenv/versions/3.6.3/lib/python3.6/urllib/parse.py", line 439, in urlsplit
    raise ValueError("Invalid IPv6 URL")
ValueError: Invalid IPv6 URL
Invalid IPv6 URL

>>> yarl.URL("https://[:x@localhost")
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/Users/hynek/.virtualenvs/ansible-tools/lib/python3.6/site-packages/yarl/__init__.py", line 149, in __init__
    val = urlsplit(val)
  File "/Users/hynek/.pyenv/versions/3.6.3/lib/python3.6/urllib/parse.py", line 439, in urlsplit
    raise ValueError("Invalid IPv6 URL")
ValueError: Invalid IPv6 URL
Invalid IPv6 URL

Sadly, the error seems to come from the stdlib tho:

>>> p.urlsplit("https://x:[@localhost")
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/Users/hynek/.pyenv/versions/3.6.3/lib/python3.6/urllib/parse.py", line 439, in urlsplit
    raise ValueError("Invalid IPv6 URL")
ValueError: Invalid IPv6 URL
Invalid IPv6 URL