issue while using "unique: true" for the sequence data type

### Environment

* Python version: 3.5.2
* PyKwalify version: 1.8.0

### Steps to Reproduce

1. create input YML file as below
test.yml
---
mydata:
name: 123
age: ABCD
hobbies: [cooking, painting, cooking]

2. create YML schema as below
---
```
type: map
mapping:
mydata:
type: map
mapping:
name:
type: str
range:
min: 1
max: 10
age:
type: int
range:
min: 21
max: 100
hobbies:
type: seq
sequence:
- type: str
unique: true
required: true
```

3. Write Python test case for validation as below
validate_yml.py

```
#! /usr/bin/python3

@author: Minal Deshmukh

import pykwalify
from pykwalify.core import Core, SchemaError
from pykwalify.errors import RuleError

yaml_file = "./test.yml"
schema_file = "./schema.yml"
yaml_schema = Core(source_file=yaml_file, schema_files=[schema_file])
try:
pykwalify.init_logging(0)
yaml_schema.validate(raise_exception=True)
except (SchemaError, RuleError):
for error in yaml_schema.errors:
print("error -->",error)
print ("Data Type of error is -->",type(error))
```

4. Execute test case as below
python3 validate_yml.py

OUTPUT

```
error --> Value 'ABCD' is not of type 'int'. Path: '/mydata/age'
Data Type of error is -->
error --> Value '123' is not of type 'str'. Path: '/mydata/name'
Data Type of error is -->
error --> Value 'cooking' is not unique. Previous path: '/mydata/hobbies/0'. Path: '/mydata/hobbies/2'
Data Type of error is -->
```

### Schema

```
type: map
mapping:
mydata:
type: map
mapping:
name:
type: str
range:
min: 1
max: 10
age:
type: int
range:
min: 21
max: 100
hobbies:
type: seq
sequence:
- type: str
unique: true
required: true
```

### Data

```
mydata:
name: 123
age: ABCD
hobbies: [cooking, painting, cooking]
```

### Expected Behavior
Expected data type of the error below is 'pykwalify.errors.SchemaError.SchemaErrorEntry'

error --> Value 'cooking' is not unique. Previous path: '/mydata/hobbies/0'. Path: '/mydata/hobbies/2'
Data Type of error is -->

### Observed Behavior
Data type is different than the other error data types
error --> Value 'cooking' is not unique. Previous path: '/mydata/hobbies/0'. Path: '/mydata/hobbies/2'
Data Type of error is -->