[CQT-89] Version should be defined in a single place

[rturrado]

When we started using cibuildwheels to generate the wheels, we also started using a pyproject.toml file (to express some cibuildwheel options). And we updated setup.py.

However, one of the things we couldn't get working was to keep setup.py querying the version from version.py. The from version import get_version in setup.py wasn't working anymore with the introduction of pyproject.toml.

The quick and dirty solution to this was to copy-paste the get_version function from version.py to setup.py.

There are other places where we use a hardcoded version:

• emscripten/test_libqasm.ts: a Typescript code that queries the version and compares it to a hardcoded string.
• README.md: when showing how to use conan create, which receives a version parameter.