chore(api): bump Python

Other occurrences of `.__func__` are on managed classes and thus can't
be removed without necessitating a new Django migration.

api/Dockerfile

@@ -1,4 +1,4 @@
-FROM python:3.9-alpine
+FROM python:3.10-alpine
 
 COPY --from=trajano/alpine-libfaketime /faketime.so /lib/libfaketime.so
 RUN mkdir -p /etc/faketime

api/desecapi/models.py

@@ -573,14 +573,14 @@ class Donation(ExportModelOperationsMixin('Donation'), models.Model):
         MONTHLY = 1
         QUARTERLY = 3
 
-    created = models.DateTimeField(default=_created_default.__func__)
+    created = models.DateTimeField(default=_created_default)
     name = models.CharField(max_length=255)
     iban = models.CharField(max_length=34)
     bic = models.CharField(max_length=11, blank=True)
     amount = models.DecimalField(max_digits=8, decimal_places=2)
     message = models.CharField(max_length=255, blank=True)
-    due = models.DateTimeField(default=_due_default.__func__)
-    mref = models.CharField(max_length=32, default=_mref_default.__func__)
+    due = models.DateTimeField(default=_due_default)
+    mref = models.CharField(max_length=32, default=_mref_default)
     interval = models.IntegerField(choices=Interval.choices, default=Interval.ONCE)
     email = models.EmailField(max_length=255, blank=True)