Support for Python 2.x

[snorfalorpagus]

Python 2.7 is reaching End Of Life and will no longer be supported by PSF from 1st January, 2020 (11 months from now).

• https://www.python.org/dev/peps/pep-0373/
• Update Python 2.7 EOL date python/devguide#344

We should have a plan for how we're going to handle support in Fiona (and related projects).

Could Fiona 1.8 be the last release that supports 2.x?

Relevant to #715.

[snowman2]

Should a similar issue be created for rasterio?

[sgillies]

@snorfalorpagus what would you think about dropping 2.7 from the appveyor build matrix instead of spending time solving that issue?

Update: we're now ignoring build failures involving Python 2.7 which happen all the time due to an interaction between powershell and Python 2.7 deprecation notices that I do not understand (the interaction, that is, not the deprecation).

[sgillies]

Python 2.7 has been removed from both Appveyor and Travis-CI configuration files.