guess_types turns negative percentages into positive
*Created originally on Bitbucket by [mdemenok (Mike Demenok)](https://bitbucket.org/%7B2bcc7b66-d9a6-4bc3-81bc-35a1e3e6311b%7D/)*
The regex used to parse percentages out of strings currently drops the minus sign, turning negative numbers into positive.
Reproducible case:
```
#!python
import openpyxl, re
wb = openpyxl.Workbook(guess_types=True)
ws = wb.active
ws.cell(row=1, column=1, value="Broken value")
cell = ws.cell(row=1, column=2, value="-20%")
print("This should be negative, but isn't: {}".format(cell.value))
openpyxl.cell.cell.PERCENT_REGEX = re.compile(r'^(?P<number>-?[0-9]*\.?[0-9]*\s?)\%$')
ws.cell(row=2, column=1, value="Correct value")
cell = ws.cell(row=2, column=2, value="-20%")
print("This should be negative, and is: {}".format(cell.value))
wb.save("negative_percentages.xlsx")
```
It's a simple fix by moving the - sign into the capture group, i.e. change from:
```
#!python
PERCENT_REGEX = re.compile(r'^\-?(?P<number>[0-9]*\.?[0-9]*\s?)\%$')
```
to
```
#!python
PERCENT_REGEX = re.compile(r'^(?P<number>-?[0-9]*\.?[0-9]*\s?)\%$')
```
Please note that same problem exists in NUMBER_REGEX, but the capture group result is never actually used. Consider updating it as well in case this changes in the future.
*Attachments:* [openpyxl_819_repr.py](/uploads/86d72ee8b1886549c8b48202d3554d9c/openpyxl_819_repr.py)
issue