How can I get knitr to output python 3 code and make it available in R chunks?

Question

I have the following example markdown where I load an environment en with python 3.7, numpy 1.19.2 and dependencies:

    title: "test knitr python"
    output: html_document
    ---

    ```{r setup, include=FALSE}
    library(reticulate)
    use_condaenv('en')  # Tell R which python environment to use. 
    knitr::opts_chunk$set(echo = TRUE)
    ```

    ```{python}
    a = 1
    ```

    ```{r}
    py$a
    ```

which outputs the expected [1] 1 when I Run All in RStudio. However, when I try to knit the code to html, the output in the R Markdown console runs up to the python chunk and then stalls there without error message and no html is produced. This is the last line in the R Markdown console: $ engine: chr "python".

I use the following software:

• Mac OS 10.15
• RStudio 1.3
• knitr 1.30
• reticulate 1.18
• python 3.7.9
• numpy 1.19.2

reticulate::py_config shows the expected environment.

Here's what I tried so far:

• Searching the reticulate and knitr documentation for examples specific to python3 (found none so I assume it should in principle be straight forward)
• Searched for similar questions on stack overflow. I followed up on all of the following Q&As and comments and tried out suggested solutions but either wasn't able to figure out how the answer could solve my problem (e.g. here) or I wasn't able to get my minimal example to work properly with the suggested answers (e.g. here, adding use_python in the setup chunk or specifying engine.path in the python chunk). All resulted in the same behaviour in the R Markdown console.
• Tried to run the code with the default python reticulate would choose if I do not specify an environment by restarting the R session and commenting out the use_conda line. reticulate::py_config uses python 2.7 and trows the following error when reaching the python chunk:

Error in py_call_impl(callable, dots$args, dots$keywords) : 
  AttributeError: 'module' object has no attribute '__import__'

Detailed traceback: 
  File "/Library/Frameworks/R.framework/Versions/4.0/Resources/library/reticulate/python/rpytools/loader.py", line 13, in initialize
    _import = builtins.__import__

There seems to be a problem when loading python. I cannot explain this error message but it may be related with why I cannot get the markdown example above to run.

I'd very much appreciate some hints about how I can proceed to get this code running.