pycriu: fix lint errors

This patch fixes the following errors reported by ruff:

lib/pycriu/images/pb2dict.py:307:24: E721 Use `is` and `is not` for type comparisons, or `isinstance()` for isinstance checks
    |
305 |     elif field.type in _basic_cast:
306 |         cast = _basic_cast[field.type]
307 |         if pretty and (cast == int):
    |                        ^^^^^^^^^^^ E721
308 |             if is_hex:
309 |                 # Fields that have (criu).hex = true option set
    |

lib/pycriu/images/pb2dict.py:379:13: E721 Use `is` and `is not` for type comparisons, or `isinstance()` for isinstance checks
    |
377 |     elif field.type in _basic_cast:
378 |         cast = _basic_cast[field.type]
379 |         if (cast == int) and is_string(value):
    |             ^^^^^^^^^^^ E721
380 |             if _marked_as_dev(field):
381 |                 return encode_dev(field, value)
    |

Signed-off-by: Radostin Stoyanov <rstoyanov@fedoraproject.org>

lib/pycriu/images/pb2dict.py

@@ -304,7 +304,7 @@ def _pb2dict_cast(field, value, pretty=False, is_hex=False):
         return field.enum_type.values_by_number.get(value, None).name
     elif field.type in _basic_cast:
         cast = _basic_cast[field.type]
-        if pretty and (cast == int):
+        if pretty and cast is int:
             if is_hex:
                 # Fields that have (criu).hex = true option set
                 # should be stored in hex string format.
@@ -376,7 +376,7 @@ def _dict2pb_cast(field, value):
         return field.enum_type.values_by_name.get(value, None).number
     elif field.type in _basic_cast:
         cast = _basic_cast[field.type]
-        if (cast == int) and is_string(value):
+        if cast is int and is_string(value):
             if _marked_as_dev(field):
                 return encode_dev(field, value)