Interface cases function with SymPy's piecewise

[rwst]

In #20191 the cases function is introduced. This ticket should interface it with SymPy's piecewise (both directions) and test with:

        sage: _ = var('y, z')
        sage: (x^y - z).integrate(y)
        -y*z + x^y/log(x)
        sage: (x^y - z).integrate(y, algorithm="sympy")  # see Trac #14694
        -y*z + cases(((log(x) == 0, y), (1, x^y/log(x))))

Depends on #20191
Depends on #23990
Depends on #24006

CC: @mforets @EmmanuelCharpentier

Component: interfaces

Author: Ralf Stephan

Branch/Commit: b261ec3

Reviewer: Emmanuel Charpentier

Issue created by migration from https://trac.sagemath.org/ticket/23923

[rwst]

Changed dependencies from #20191 to #20191, #23990

[rwst]

Branch: u/rws/interface_cases_function_with_sympy_s_piecewise

[EmmanuelCharpentier]

Commit: ea10ee7

[EmmanuelCharpentier]

New commits:

ea10ee7

23923: interface cases() with SymPy

[rwst]

Changed branch from u/rws/interface_cases_function_with_sympy_s_piecewise to u/rws/23923

[sagetrac-git]

Changed commit from ea10ee7 to b80d98c

[sagetrac-git]

Branch pushed to git repo; I updated commit sha1. New commits:

b80d98c

23923: fix typo

[rwst]

Author: Ralf Stephan

[rwst]

Changed dependencies from #20191, #23990 to #20191, #23990, #24006

[EmmanuelCharpentier]

comment:7

On top of 8.1beta7+#24026 + #24006(dependency) + #23990(dependency) + #23923(present patch), one permanent failure :

sage -t --long src/sage/symbolic/integration/integral.py
**********************************************************************
File "src/sage/symbolic/integration/integral.py", line 508, in sage.symbolic.int
egration.integral.integrate
Failed example:
    (x^y - z).integrate(y, algorithm="sympy")  # see Trac #14694
Expected:
    Traceback (most recent call last):
    ...
    AttributeError: 'ExprCondPair' object has no attribute '_sage_'
Got:
    -y*z + cases(((log(x) == 0, y), (1, x^y/log(x))))
**********************************************************************
1 item had failures:
   1 of 126 in sage.symbolic.integration.integral.integrate
    [169 tests, 1 failure, 53.41 s]

==>needs_work. Probably a booboo, though...