Encode relative path to working tree in code paths when possible.

Fix Python 2 and 3 have diffrent default encoding

Closes #1594.

- resolve string escapes  only outside of fstring expressions
- reject backslashes inside of fstring expressions
Also make some fstring errors non-fatal to keep parsing.

Allowed importing pyx files from ZIP archives

Issue a warning as this is a behavioural change that can lead to subtle code breakage of currently working code, in the same way that the previous behaviour could easily lead to bugs in general.

Resolves #1602.

ENH: update test skip list for pypy

Delete trailing spaces

use compiled Cython for appveyor build: we compile it anyway so that is what we should test (and it speeds up the tests)

Vector cleanups