Markdown-to-Markdown gives raw elements

[Geoffrey Poore]

This was discovered in looking into #25.

Input:

```{.python .cb.run}
print('''
|   Name    |  SST-2  |  MRPC  |
|-----------|---------|--------|
| accuracy  |  0.868  | 0.757  |
| buildtime | 781.831 | 50.946 |
''')
```

Convert -t markdown with Pandoc 2.9.2.1:

```{=markdown}
|   Name    |  SST-2  |  MRPC  |
|-----------|---------|--------|
| accuracy  |  0.868  | 0.757  |
| buildtime | 781.831 | 50.946 |
```

The ```{=markdown} shouldn't be there, so there is probably an incompatibility with something changed in a recent version of Pandoc.