Fix #191: improve handling of dependencies

[Mateusz-Grzelinski]

This PR will allow for update/downgrade of dependencies when needed.

• do not use import as a way of checking for version or checking if module can be imported. When Windows opens file it can not longer be removed! The same problem is being faced by blender developers: https://projects.blender.org/blender/blender/commit/b38439db99bd3791ff1dc55ddfaa9d227b1bad87
• use one pip call to install dependencies. It allows to correctly handle constraints.
• when package is outside of constraints, perform clean installation. Clean installation in most cases is not necessary but with --target it has strange corner cases.
  • fox example when you are not doing clean install, but only upgrader with pip install --upgrade you can only downgrade package...
• commit semver dependency with code

This PR should be pretty complete, can not think of anything specific to test.