- Use multibuild to avoid cycles with pytest

- Update to 16.7.5:
* fix powershell activation when sourced (#1398)
* fix regression - sh activation script not working under sh (only bash) (#1396)
* activate.ps1 syntax and style updated to follow PSStyleAnalyzer rules (#1371)
* Allow creating virtual environments for 3.xy. (#1385)
* Report error when running activate scripts directly
* Extend the LICENSE search paths list by lib64/pythonX.Y to support Linux vendors who install their Python to /usr/lib64/pythonX.Y
* Raise an error if the target path contains the operating systems path separator (using this would break our activation scripts)
* Drop Jython support
* Add tests covering prompt manipulation during activation/deactivation, and harmonize behavior of all supported shells

- Update to 16.4.3:
* Revert the symlink fix, causing debian packaging issues. (#1390)
* Fix license() builtin by copying the LICENSE file into the virtualenv - by asottile. (#1317)
* fixes the scenario where the python base install is symlinked with relative symlinks (#490)
* Use importlib over imp in virtualenv.py for python >= 3.4 - by Anthony Sottile (#1293)
* Copy or link PyPy header files instead of include directory itself (#1302)
* Allow virtualenv creation with older pip not having config command correspondingly disabling configuration related features (such as pip cert setting) in this case. (#1303)
* Use importlib over deprecated imp` in ``distutils/__init__.py for python 3 - by Anthony Sottile (#955)
* Preserve cert option defined in pip.conf or environment variable. (#1273)
* fixed a ResourceWarning: unclosed file in call_subprocess() - by Mickaël Schoentgen (#1277)
* pre-import some built-in modules in site.py on PyPy according to PyPy's site.py - by microdog (#1281)
* Copy files from sys.exec_prefix only if it is really different path than used prefix, bugfix for #1270 (#1282)
* copyfile handles relative symlinks and symlinks to symlinks, avoiding problems when Python was installed using stow or homebrew. (#268)
* Fix preserving of original path when using fish and a subshell. (#904)
* Drop the source layout of the project, going back to how the source was laid out before 16.1.0. (#1241)
* Fix bootstrap script generation broken with 16.0.0. Support now both CPython, pypy, jython. (#1244)
* lib64 symlink is again relative (as was with < 16.1.0). (#1248)