Markdown parsing bug: multi-line string with codeblock fails to roundtrip

[rsokl]

In a jupytext markdown file, opened as a notebook, the cell:

will be not be parsed correctly upon opening the md file again:

Here is the markdown file:

---
jupyter:
  jupytext:
    text_representation:
      extension: .md
      format_name: markdown
      format_version: '1.2'
      jupytext_version: 1.3.1
  kernelspec:
    display_name: Python [conda env:hardnet]
    language: python
    name: conda-env-hardnet-py
---

```python
readme = """
above

```python
x = 2
```

below
"""
```

[mwouts]

Interesting! Thanks @rsokl for spotting this. I'll have a look soon.