Feature request: Allow the subset argument of dropna and drop_duplicates to take a single label
This works:
```
df.drop(columns='My Col')
```
But neither of these do:
```
df.dropna(subset='My Col')
df.drop_duplicates(subset='My Col')
df.duplicated(subset='My Col')
```
all of which instead should be called with `subset=['My Col']`. 
This is a feature request for the three latter functions to allow the subset argument to take a single label.