dict unpacking broken for `.plot` in 3.2

Edited this issue about 14 times, here is the minimal works/doesn't:

## Works:
```python
d = {4:1, 6:2}
plt.bar(d.keys(), d.values())
```
## doesn't work
```python
d = {4:1, 6:2}
plt.plot(d.keys(), d.values())
```
doesn't work as of '3.2.1.post2558.dev0+geeeafe9c2' & support for this was added in #6787
