Exception if opening workbook with unicode in formula
*Created originally on Bitbucket by [agellhaus (Arne Gellhaus)](https://bitbucket.org/%7B2e51ecdf-b5cc-4a3b-b462-218de4fa3a3b%7D/)*
Exception: TypeError: expected <type 'str'>
Could be fixed changing openpyxl\formatting\rule.py
```
#!python
class Rule(...):
formula = Sequence(expected_type=str)
```
to
```
#!python
class Rule(...):
formula = Sequence(expected_type=basestring)
```
issue