This is the mirror image of #2170.

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     just remove those checkboxes, or mark them as checked. Otherwise,
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- [x] Add a [CHANGELOG](https://github.com/pyodide/pyodide/blob/main/docs/project/changelog.md) entry
- [x] Add / update tests
- [x] Add new / update outdated documentation

Add `default_converter` argument to `to_js` and `toJs`. This is the mirror image of #2170.

### Checklists

<!-- Note on checklists:
     If you think some of these steps are not necessary for your PR,
     just remove those checkboxes, or mark them as checked. Otherwise,
     if some checkboxes are left unmarked, we might assume that your PR
     is not ready to be merged and we might keep you waiting -->

- [x] Add a [CHANGELOG](https://github.com/pyodide/pyodide/blob/main/docs/project/changelog.md) entry
- [x] Add / update tests
- [x] Add new / update outdated documentation

From f1d8f0b92b17765a2e8b6ac508cfa77a79fb73a0 Mon Sep 17 00:00:00 2001
From: Hood Chatham <roberthoodchatham@gmail.com>
Date: Sun, 26 Dec 2021 22:39:25 -0800
Subject: [PATCH] Cythonize _cython_blas.pyx with include path pointing to
 scipy

---
 sklearn/utils/setup.py | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/sklearn/utils/setup.py b/sklearn/utils/setup.py
index ed78ecc5d..ea1a61b40 100644
--- a/sklearn/utils/setup.py
+++ b/sklearn/utils/setup.py
@@ -2,6 +2,7 @@ import os
 from os.path import join
 
 from sklearn._build_utils import gen_from_templates
+from Cython.Build import cythonize
 
 
 def configuration(parent_package="", top_path=None):
@@ -18,6 +19,7 @@ def configuration(parent_package="", top_path=None):
         "sparsefuncs_fast", sources=["sparsefuncs_fast.pyx"], libraries=libraries
     )
 
+    cythonize("sklearn/utils/_cython_blas.pyx", include_path=[f"{os.environ['PYODIDE_ROOT']}/packages/scipy/build/scipy-1.7.3/"])
     config.add_extension(
         "_cython_blas", sources=["_cython_blas.pyx"], libraries=libraries
     )
-- 
2.25.1


From f1d8f0b92b17765a2e8b6ac508cfa77a79fb73a0 Mon Sep 17 00:00:00 2001
From: Hood Chatham <roberthoodchatham@gmail.com>
Date: Sun, 26 Dec 2021 22:39:25 -0800
Subject: [PATCH] Cythonize _cython_blas.pyx with include path pointing to
 scipy

---
 sklearn/utils/setup.py | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/sklearn/utils/setup.py b/sklearn/utils/setup.py
index ed78ecc5d..ea1a61b40 100644
--- a/sklearn/utils/setup.py
+++ b/sklearn/utils/setup.py
@@ -2,6 +2,7 @@ import os
 from os.path import join
 
 from sklearn._build_utils import gen_from_templates
+from Cython.Build import cythonize
 
 
 def configuration(parent_package="", top_path=None):
@@ -18,6 +19,7 @@ def configuration(parent_package="", top_path=None):
         "sparsefuncs_fast", sources=["sparsefuncs_fast.pyx"], libraries=libraries
     )
 
+    cythonize("sklearn/utils/_cython_blas.pyx", include_path=[f"{os.environ['PYODIDE_ROOT']}/packages/scipy/build/scipy-1.7.3/"])
     config.add_extension(
         "_cython_blas", sources=["_cython_blas.pyx"], libraries=libraries
     )
-- 
2.25.1

