drop unnecessary backwards compatibility (remove six)

[robbkidd]

Beeline hasn't supported use under Python 2 for a while now, so we can remove compat code.

Which problem is this PR solving?

• Resolves Missing transitive dependency from libhoney (six) #227

Short description of the changes

• Removes uses of six

Note: six is still a transitive dep via dev dependencies, so it still appears in the poetry.lock.

» poetry show --tree
mock 3.0.5 Rolling backport of unittest.mock for all Pythons
└── six *
pylint 2.6.2 python code static checker
└── astroid >=2.4.0,<2.5
    └── six >=1.12,<2.0

[vreynolds]

🧹 ✨