Drop official support for Python 3.5

[keitherskine]

It seems about time to drop official support for Python 3.5, which is now quite old. Besides, the build tests for Python 3.5 have started failing recently, probably in part because pip dropped support for Python 3.5 back in January. Rather than jerry-rig the unit tests to get them working with 3.5, it seems easiest to just move on.

Related: #857 (note Kleehammer's comment)

[abitrolly]

Yep, @appveyor upgraded pip https://www.appveyor.com/docs/windows-images-software/#visual-studio-2019 to Python 3.5 incompatible version (while still shipping Python 3.5), and now Python 3.5 Windows builds are failing https://ci.appveyor.com/project/mkleehammer/pyodbc/builds/41154018/job/vqfa6d6ednmy8std

[abitrolly]

appveyor/build-images@b31c9c5#diff-3e246576e305dd46737b653748cdf6ac4c113d611ec567a8e8aae063d1a636dd

https://pip.pypa.io/en/stable/news/#v21-0