gh-40686: Install pyx sources with meson
    
Currently the meson build doesn't install the pyx source files, as
opposed to the setuptools build. This has caused/is causing several
issues:
- The method `sage_getsource` as well as ipython's own source viewer
`??` don't work for cython methods. This creates an inconsistency with
python methods, and breaks several tests when the sage source is not
available (eg. distro packages).
- It breaks the doc builder
#39973 (comment)
- It used to cause issues when accessing some methods
(#39735), now worked around.

The only reason why these issues are not present in sage-the-distro is
that cython wrongly embeds the absolute paths of source files, which
masks the issues as long as the source is present in its original
location. But as soon as cython/cython#6755 is
released this will affect all types of sage build.

In any case, I believe being able to inspect the methods implementation
from the sage interface itself is a valuable feature for mathematicians,
which are its main user base.

I have only done one dir so far so I don't waste my time in case this is
rejected or a different implementation is proposed. I will do the rest
when I get the OK.

Fixes #39874
    
URL: #40686
Reported by: Antonio Rojas
Reviewer(s): Tobias Diez

Author: Release Manager

src/sage/algebras/finite_dimensional_algebras/meson.build

@@ -3,6 +3,7 @@ py.install_sources(
   'all.py',
   'finite_dimensional_algebra.py',
   'finite_dimensional_algebra_element.pxd',
+  'finite_dimensional_algebra_element.pyx',
   'finite_dimensional_algebra_ideal.py',
   'finite_dimensional_algebra_morphism.py',
   subdir: 'sage/algebras/finite_dimensional_algebras',

src/sage/algebras/fusion_rings/meson.build

@@ -3,11 +3,15 @@ py.install_sources(
   'all.py',
   'f_matrix.py',
   'fast_parallel_fmats_methods.pxd',
+  'fast_parallel_fmats_methods.pyx',
   'fast_parallel_fusion_ring_braid_repn.pxd',
+  'fast_parallel_fusion_ring_braid_repn.pyx',
   'fusion_double.py',
   'fusion_ring.py',
   'poly_tup_engine.pxd',
+  'poly_tup_engine.pyx',
   'shm_managers.pxd',
+  'shm_managers.pyx',
   subdir: 'sage/algebras/fusion_rings',
 )
 

src/sage/algebras/letterplace/meson.build

@@ -2,7 +2,10 @@ py.install_sources(
   '__init__.py',
   'all.py',
   'free_algebra_element_letterplace.pxd',
+  'free_algebra_element_letterplace.pyx',
   'free_algebra_letterplace.pxd',
+  'free_algebra_letterplace.pyx',
+  'letterplace_ideal.pyx',
   subdir: 'sage/algebras/letterplace',
 )
 

src/sage/algebras/lie_algebras/meson.build

@@ -13,6 +13,7 @@ py.install_sources(
   'heisenberg.py',
   'lie_algebra.py',
   'lie_algebra_element.pxd',
+  'lie_algebra_element.pyx',
   'morphism.py',
   'nilpotent_lie_algebra.py',
   'onsager.py',

src/sage/algebras/meson.build

@@ -8,10 +8,12 @@ py.install_sources(
   'cellular_basis.py',
   'clifford_algebra.py',
   'clifford_algebra_element.pxd',
+  'clifford_algebra_element.pyx',
   'cluster_algebra.py',
   'commutative_dga.py',
   'down_up_algebra.py',
   'exterior_algebra_groebner.pxd',
+  'exterior_algebra_groebner.pyx',
   'finite_gca.py',
   'free_algebra.py',
   'free_algebra_element.py',
@@ -24,6 +26,7 @@ py.install_sources(
   'jordan_algebra.py',
   'nil_coxeter_algebra.py',
   'octonion_algebra.pxd',
+  'octonion_algebra.pyx',
   'orlik_solomon.py',
   'orlik_terao.py',
   'q_commuting_polynomials.py',

src/sage/algebras/quatalg/meson.build

@@ -2,7 +2,9 @@ py.install_sources(
   '__init__.py',
   'all.py',
   'quaternion_algebra.py',
+  'quaternion_algebra_cython.pyx',
   'quaternion_algebra_element.pxd',
+  'quaternion_algebra_element.pyx',
   subdir: 'sage/algebras/quatalg',
 )
 

src/sage/arith/meson.build

@@ -3,12 +3,18 @@ py.install_sources(
   'all.py',
   'constants.pxd',
   'functions.pxd',
+  'functions.pyx',
   'long.pxd',
   'misc.py',
   'multi_modular.pxd',
+  'multi_modular.pyx',
   'numerical_approx.pxd',
+  'numerical_approx.pyx',
   'power.pxd',
+  'power.pyx',
   'rational_reconstruction.pxd',
+  'rational_reconstruction.pyx',
+  'srange.pyx',
   subdir: 'sage/arith',
 )
 

src/sage/calculus/meson.build

@@ -6,11 +6,17 @@ py.install_sources(
   'expr.py',
   'functional.py',
   'functions.py',
+  'integration.pyx',
   'interpolation.pxd',
+  'interpolation.pyx',
+  'interpolators.pyx',
   'ode.pxd',
+  'ode.pyx',
   'predefined.py',
+  'riemann.pyx',
   'test_sympy.py',
   'tests.py',
+  'var.pyx',
   'wester.py',
   subdir: 'sage/calculus',
 )

src/sage/calculus/transforms/meson.build

@@ -3,7 +3,9 @@ py.install_sources(
   'all.py',
   'dft.py',
   'dwt.pxd',
+  'dwt.pyx',
   'fft.pxd',
+  'fft.pyx',
   subdir: 'sage/calculus/transforms',
 )
 

src/sage/categories/examples/meson.build

@@ -29,6 +29,7 @@ py.install_sources(
   'monoids.py',
   'posets.py',
   'semigroups.py',
+  'semigroups_cython.pyx',
   'semirings.py',
   'sets_cat.py',
   'sets_with_grading.py',