chore: Drop support for Python 3.6, check compatibility with Python 3.10

[phanak-sap]

Python 3.6 reached EOL (no security fixes) in 23 Dec 2021 and there is no actual need to stay backward-compatible. The lowest python version or this package should now be 3.7.

• Add Python 3.10 to build matrix - chore: Update .travis.yml - Add Python 3.10 and latest LXML #185
• Drop Python 3.6 from travis builds - chore: Drop support for Python 3.6, check compatibility with Python 3.10 #190
• Update the setup.py ( pipy documentation), update readme - chore: Drop support for Python 3.6, check compatibility with Python 3.10 #190
• consider lowest LXML version possible to support based on 3.6 drop, update travis and setup.py dependencies accordingly - chore: Drop support for Python 3.6, check compatibility with Python 3.10 #190

[phanak-sap]

all done, in version 1.8.0