`poetry build` creates a whl file using `os.mkstemp`. The file is
created with very restrictive permissions. This commit ensures these are
normalized before the build starts.

Partly addresses https://github.com/sdispater/poetry/issues/671.

- Fix non-sha256 hashes retrieval
- Fix resolution of packages with Python 3 only wheel

* use symlinks in created venv by default on non-windows platforms. Issue #779

* Fixed test for deployments without venv module available.