Parameter strings cannot contain "="

## 🐛 Bug

```python
# %% tags=["parameters"]

s = "a=b"

# %%
print(s)
```
This fails with: `Passed unknown parameter: `s`".
Seems like papermill naively counts = on the parameter line, so it's treated as unparseable, then flagged as “unknown parameter”.

Papermill version: `2.6.0`.