Remove conditional for beautifulsoup4 < 4.7
Remove conditional for beautifulsoup4 < 4.7

Since 58a8149b9, we have required beautifulsoup4 >= 4.7, so we do not
need to retain the code for prior versions in our testing utilities.

This also fixes the Python 3.10 deprecation warning:

> DeprecationWarning: distutils Version classes are deprecated.
> Use packaging.version instead.

MechanicalSoup/MechanicalSoup