build(deps): bump juliacall from 0.9.20 to 0.9.21

Bumps [juliacall](https://github.com/JuliaPy/PythonCall.jl) from 0.9.20 to 0.9.21.
- [Release notes](https://github.com/JuliaPy/PythonCall.jl/releases)
- [Commits](JuliaPy/PythonCall.jl@v0.9.20...v0.9.21)

---
updated-dependencies:
- dependency-name: juliacall
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

Signed-off-by: dependabot[bot] <support@github.com>

requirements.txt

@@ -2,6 +2,6 @@ sympy>=1.0.0,<2.0.0
 pandas>=0.21.0,<3.0.0
 numpy>=1.13.0,<3.0.0
 scikit_learn>=1.0.0,<2.0.0
-juliacall==0.9.20
+juliacall==0.9.21
 click>=7.0.0,<9.0.0
 setuptools>=50.0.0