`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.

Adresses #742 and #239

https://docs.python.org/2/library/zipfile.html#zipfile.ZipFile.writestr

Note: When passing a ZipInfo instance as the zinfo_or_arcname parameter,
the compression method used will be that specified in the compress_type
member of the given ZipInfo instance. By default, the ZipInfo
constructor sets this member to ZIP_STORED.

* fixing #724

* added test for issue 724