How to build sage with python3:

make configure
./configure --with-python=3
make

Beware that the second step above should be redone after every make distclean.

For progress on making all doctests pass, see #26212 and #28298

In order to support python 3.7, the following needs to be fixed:

1. upgrade setuptools to version 2.0.1 #15510 - upgrade to a newer version of setuptools
2. upgrade rpy2 to 2.3.8 #15511 - upgrade to a newer version of rpy2
3. upgrade sympy to version 0.7.4 #15512 - upgrade to a newer version of sympy
4. Update pycrypto to 2.6.1 #14854 - upgrade to a newer version of pycrypto
5. upgrade networkx to version 1.8.1 #15532 - upgrade to a newer version of networkx
6. fix csage to build against python3 #15537 - fix csage to work with python3
7. use Pillow instead of PIL #15539 - switch from using PIL to Pillow
8. trivial pkg fixes needed for python3 #15540 - trivial python3 fixes to a few spkgs
9. sage-location and sage-download-file are broken with python3 #15541 - fix sage-location and sage-download-file for python3
10. remove sqlalchemy #15593 - remove sqlalchemy
11. remove gdmodule #17591 - remove gdmodule
12. StandardError is not Python 3 compatible #15620 - Stop using StandardError
13. Upgrade cython to version 0.20.1 #15755 - upgrade cython to version 0.20.1
14. Upgrade mpmath to version 0.18 #15807 - upgrade mpmath to version 0.18
15. Meta-ticket for Python 3.6+ preparation (sage library) - stage 1 #15980 - meta-ticket for python3 compatibility of the sage library (stage 1)
16. Meta-ticket for Python 3.6+ preparation (sage library) - stage 2 #16052 - meta-ticket for python3 compatibility of the sage library (stage 2)
17. Metaticket: remove c_lib #17854 - remove c_lib
18. Switch from PolyBoRi to BRiAl #18437 - fix polybori and python 3
19. upgrade to pynac-0.3.9.1 #18537 - fix Pynac at least building against python 3
20. Add a python 3 package #17607 - add python 3 package
21. update sagetex to version 3.2 #27024 - make sagetex python3 compatible

Possibly relevant timeline from PEP 373:

Being the last of the 2.x series, 2.7 will have an extended period of maintenance. The current plan is to support it for at least 10 years from the initial 2.7 release. This means there will be bugfix releases until 2020.

Depends on #15510
Depends on #15511
Depends on #15512
Depends on #15531
Depends on #15532
Depends on #15537
Depends on #15539
Depends on #15540
Depends on #15541
Depends on #15807
Depends on #15980
Depends on #18537
Depends on #16052
Depends on #18437
Depends on #27024

Component: python3

Reviewer: Dima Pasechnik

Issue created by migration from https://trac.sagemath.org/ticket/15530