We are going to depend on packaging for converting to a PEP 440
compliant version.

Move a slightly modified version of get_version_from_pyproject_toml
function to the utils module.

The pyproject.toml file is **NOT** contained in the distributed python
package and therefore the version can't be loaded from the
pyproject.toml file at all.

As a result the version information must be set at two places

1. In the [tool.poetry] section of the pyproject.toml file
2. In a file within the installed and distributed python package

Convert verify-version.py into a more general utility to handle the
version information in python-gvm.

Allow to call the module via python -m

These function as version relates thus they should be in the new version
module.

Allow to reuse some of the code in external libraries and tools.

Also don't crash if version is not contained in [tool.poetry] section.

Rename check_version_is_pep440_compliant to is_version_pep440_compliant
and add a testcase.

Don't add extra spaces at the end of the generated __version__.py file.

tomlkit allos to preserver the current structure and layout when saving
the toml file.

Update the pyproject.toml version by reading and writing the file
directly instread of calling poetry update.