Checks

• I added a descriptive title to this issue
• I have searched (google, github) for similar issues and couldn't find anything
• I have read and followed the docs and still think this is a bug

Bug

Output of python -c "import pydantic.utils; print(pydantic.utils.version_info())":

             pydantic version: 1.7
            pydantic compiled: True
                 install path: /root/.local/lib/python3.8/site-packages/pydantic
               python version: 3.8.6 (default, Oct 13 2020, 20:49:19)  [GCC 8.3.0]
                     platform: Linux-3.10.0-1127.19.1.el7.x86_64-x86_64-with-glibc2.2.5
     optional deps. installed: ['email-validator']

from typing import Pattern
from pydantic import BaseModel

class Model(BaseModel):
    filter: Pattern

Model(filter="^regex$").json()

This produce an error :
TypeError: Object of type 'Pattern' is not JSON serializable

I had to write a custom encoder to be able to serialize properly a Pattern type :

from typing import Pattern
from pydantic import BaseModel
import re

class Model(BaseModel):
    filter: Pattern
    class Config:
        json_encoders = {
            re.Pattern: lambda v: v.pattern,
        }

Model(filter="^regex$").json()