how to search by pair of columns
Here is an example (tested with mariadb 10.6):

```python
class Reg(Model):
    key = CharField()
    value = TextField()

Reg.insert_many([('k%02d' % i, 'v%02d' % i) for i in range(100)]).execute()
cols = Tuple(Reg.key, Reg.value)
values = [('k02', 'v02'), ('k04', 'v04'), ('k06', 'v06')]
query = (Reg.select()
         .where(cols.in_(values))
         .order_by(Reg.key))
for reg in query:
    print(reg.key, reg.value)

# Prints:
# k02 v02
# k04 v04
# k06 v06
```
How can I write something like this using peewee in mysql?
```
SELECT whatever 
FROM t
WHERE (col1, col2)
       IN ((val1a, val2a), (val1b, val2b), ...) ;
```