Getversion

Fixed bug (incorrect version number) happening when a package is both installed and available on python path. This typically happens when a developer is working on a new version of a package while an older version is already installed. Fixes [3](https://github.com/smarie/python-getversion/issues/3).

`__version__` should now be available on the distributed `getversion` package.

Simplified design: now a single `get_module_version` function is the entry point and compiles 4 strategies:

- `__version__` attribute
- built-in module using `stdlib_list`
- unzipped wheel & unzipped egg
- git scm using `setuptools_scm`

First version extracted from private industrial project. Most important legacy changelog entries for reference:

* Fixed a version detection bug happening when there is a version conflict in the pip environment (for example when package A depends on package B with version `<=1.0.0`, and the found B has version `1.1.0`).
* The version is now correctly detected even if it is a source project and an old egg-info folder with wrong version is present in filesystem.
* Bug fix: reading version from unzipped wheel
* Various bug fixes in module version handling
* Improved version detection algorithm when relying on git, in `get_pkg_version_from_module`
* new method `get_pkg_version_from_module` in `utils_modules.py`
* new module `utils_version_handling.py` for everything related to versions.