Pplpy

- switch from github to gitlab
- some more support for PPL objects: permute_space_dimensions, Bit_Row,
Bit_Matrix
- upgrade gmpy2 version dependency
- add pyproject.toml and setup.cfg files
- remove the OK functions which caused trouble for clang on FreeBSD
- switch back to github under SageMath organization
- fast initialization of Linear_Expression from list/dict
- CI (test, linter, doc build and deployment)
- add a Cython pool for ppl.Variable
- support for congruences

- use of gmpy2 for integers and rationals (Vincent Delecroix/Vincent Klein)
- tests on travis (Vincent Delecroix/Vincent Klein)
- fixes in install scripts (Vincent Delecroix)
- port some upgrades from Sage (Vincent Klein)
- split in several Cython files (Vincent Klein)
- adding widening (Jesús Doménech)