move pexpect-maxima code to maxima_lib

[rws]

This metaticket tracks efforts to remove the usage of pexpect for calling maxima, either by using maxima_lib or replacing it altogether. Code that is dependent on the maxima expect interface is in:

• calculus/desolvers.py: desolve_*()
• combinat/combinat.py: euler_number() (see #17770 and #20763)
• geometry/lattice_polytope.py: positive_integer_relations() #20766
• functions/orthogonal_polys.py: hermite() (#20297), jacobi_P(), laguerre() (#17151), legendre_P/Q() (#16813), ultraspherical() (#20428)
• functions/piecewise.py: piecewise.convolution()
• matrix/matrix1.pyx: Matrix._maxima_init_()
• matrix/matrix_symbolic_dense.pyx: Matrix_symbolic_dense.exp()
• structure/sage_object.pyx: SageObject._maxima_(), _maxima_init_()
• symbolic/assumptions.py: GenericDeclaration.assume()
• symbolic/expression.pyx: Ex.assume(), forget(), assume_str(), assert(), expand_trig(), reduce_trig(), coefficients() and poly() (both #20455), maxima_methods(), rectform(), simplify(), simplify_real(), simplify_rational(), simplify_log(), expand_log(), factor(), solve(), sum()
• symbolic/maxima_wrapper.py: *

This can be considered part of the metaticket #16688. See also ​http://trac.sagemath.org/wiki/symbolics/maxima