OverflowError when converting python int to BigInt

[programmerjake]

Related: #622 #543 #608

I think what's happening is that the conversion code is not accounting for the converted bytes needing a sign bit so allocates too few bytes when converting numbers from i8_max to u8_max and all larger multiples of 8 bits

🌍 Environment

• Your operating system and version:
  Ubuntu 18.04
• Your python version:
  Python 3.6.8
• How did you install python (e.g. apt or pyenv)? Did you use a virtualenv?:
  apt + virtualenv
• Your rust version (rustc --version):
  rustc 1.40.0-nightly (e413dc36a 2019-10-14
• Are you using the latest pyo3 version? Have you tried using latest master (replace version = "0.x.y" with git = "https://github.com/PyO3/pyo3")?
  using commit c7f3cde

💥 Reproducing

https://github.com/programmerjake/pyo3_bigint_overflow/tree/7e03b67ef43976e01950811039cc78e1490973ad

Build:

maturin develop

Test:

python -m unittest