Drop Python 3.6 support

[lemon24]

Why:

• the last 3.6 bugfix release was in December 2018, and the last security release will be in 2021
• PyPy supports 3.7 (#234)
• Windows and macOS users can install pretty much any Python version either from python.org or via Homebrew
• the latest Ubuntu LTS release came out over a year ago; the default Python 3 version is 3.8
• finally, once I migrate my own deployment to the latest Ubuntu LTS, I don't need 3.6 support anymore (done as 2021-06-08)

We can remove various things needed to support 3.6; most importantly, the dependency on the dataclasses backport.

We might be able to use structlog internally (revisit #147), due to https://www.structlog.org/en/stable/contextvars.html. Update: TBH, this still seems hard to do.

[lemon24]

To do:

• [x] add PyPy 3.7 to CI and tox
• [x] remove 3.6 from CI and tox
• [x] remove 3.6 classifier
• [x] remove dataclasses install_requires
• [x] update Python version in docs (index, install)
• [x] add changelog entry
• [x] (maybe) remove reader._utils.nullcontext (if we leave it, it would mostly still work with 3.6, since we don't use any of the other 3.7+ features)