Use CSS Table Module Level 3 to compute widths

[kygoh]

CSS Table Module Level 3 appears to respect table width over cell width. Thus, the following 3 test_layout_table_auto_9 and test_layout_table_auto_32 tests which increase table widths fail:

==================================================================== short test summary info =====================================================================
FAILED tests/layout/test_table.py::test_layout_table_auto_9 - assert 53.33333333333333 == 60
FAILED tests/layout/test_table.py::test_layout_table_auto_10 - assert 6.666666666666666 == 6
FAILED tests/layout/test_table.py::test_layout_table_auto_32 - assert 500.0 == 600
===================================================== 3 failed, 2189 passed, 39 xfailed in 323.50s (0:05:23) =====================================================

This appear to be able to fix #1956

[kygoh]

CSS Table Module Level 3 appears to respect table width over cell width.

From 3.9.2.1. Rules:

Rule 3: The sum of width assgined to all columns should be equal to the assignable table width.

[liZe]

Thanks a lot for taking the time to dive into the spec!