Improve setup module for etesync-dav

[sergeyklay]

• Remove no longer needed _version.py file
• Amend __init__.py to describe the top-level module for etesync-dav.
• Refactor setup.py to
  • avoid usage of hardcoded module info
  • avoid using exec to get version info
  • do not run setup() if __name__ is not __main__
  • provide useful helpers to retrieve package info from __init__.py
  • check if a version string is in the canonical format of PEP 440
  • add project URLs
• Improved package description generation
• Provide minimal setuptools load-test (see ci.yml)
• Provide minimal load test for Python v3.6 - v3.10 on Windows/macOs/Linux (see ci.yml)

[sergeyklay]

/cc @tasn