BUG: no attribute when the column name contains an "ϵ"
#### Code Sample, a copy-pastable example if possible

```python
df = pd.DataFrame({'col1': [0, 1]})
df['α'] = df['col1']
df['ϵ'] = df['col1']
```

This line is executed correctly:
```python
df.α
```
but this one yields `AttributeError: 'DataFrame' object has no attribute 'ε'`:
```python
df.ϵ
```

#### Problem description

With pandas 0.24.2, dataFrame columns can often be accessed as attributes, except when they contain specific unicode characters (at least "ε"). I managed to reproduce this issue when loading a CSV file with an ε inside the header, but _sometimes_ not when creating a DataFrame object from a dictionary:

This yields no exception when accessing `df.ϵ`:
```
df = pd.DataFrame(dict(α=[0, 1], ϵ=[0, 1]))
```
but this one does:
```
df = pd.DataFrame({'α': [0, 1], 'ϵ': [0, 1]})
```

