Azure Pipelines: Access matrix variable from an expression

In Azure Pipelines I'm trying to modify the value of a matrix variable with an expression as shown below.

I saw the example in How to reference an Azure DevOps matrix variable, inside an expression? but it only shows how to access a variable, not how to modify the value in an expression.

stages:
- stage: Test
  jobs:
  - job: Test
    strategy:
      matrix: 
        'Python 3.9':
          image: 'ubuntu-20.04'
          python_version: '3.9'
          pytag: 'py39'
        'Python 3.10':
          image: 'ubuntu-20.04'
          python_version: '3.10'
          pytag: 'py310'

    pool:
      vmImage: $(image)

    variables:
      var_a: $[replace(variables['python_version'], '.', '')]

    steps:
    - script: |
        echo python_version '$(python_version)'
        echo pytag $(pytag)
        echo var_a $(var_a)
        echo expression $[replace(variables['python_version'], '.', '')]
      displayName: 'Output Test'

I'm trying to get rid of the pytag variables in the matrix and to compute them within the pipeline. I would expect the output to contain env_a py39, but I receive the following output.

python_version 3.9
pytag py39
env_a
/home/vsts/work/_temp/xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx.sh: line 5: replace(variables['python_version'], '.', ''): syntax error in expression (error token is "(variables['python_version'], '.', '')")
##[error]Bash exited with code '1'.