FeedGenerator 2.1.0

• Modernize and improve tests (#32 & #34 — thanks to @venthur)
• Drop support for Python 3.6 and test on 3.10 & 3.11 (#35 — thanks to @hugovk)
• Exclude tests_feedgenerator/__pycache__ from distribution (#33 — thanks to @BenSturmfels)