Possible regression of __modify_schema__ on master

[therefromhere]

Bug

I think #1422 causes an unintentional (?) change in the behaviour of __modify_schema__ - it's now applying to the list schema as well as the item schema.

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

             pydantic version: 1.5.1
            pydantic compiled: False
                 install path: /home/johnc/Projects/pydantic/pydantic
               python version: 3.7.7 (default, Apr 18 2020, 02:59:53)  [GCC 9.3.0]
                     platform: Linux-5.4.0-29-generic-x86_64-with-Ubuntu-20.04-focal
     optional deps. installed: []

Actually at git version 5195e55 -

pip freeze | grep pydantic
-e git+https://github.com/samuelcolvin/pydantic.git@5195e55c102e537be6c00579df8f464ecd90b5ff#egg=pydantic

The output of the following code is different between master and v1.5.1

from typing import List

import pydantic


class MyField(str):
  @classmethod
  def __modify_schema__(cls, field_schema):
    field_schema["foo"] = "buzz"


class Foo(pydantic.BaseModel):
  a_field: MyField

  some_fields: List[MyField]

print(Foo.schema_json(indent=2))

With 1.5.1 this outputs:

{
  "title": "Foo",
  "type": "object",
  "properties": {
    "a_field": {
      "title": "A Field",
      "type": "string",
      "foo": "buzz"
    },
    "some_fields": {
      "title": "Some Fields",
      "type": "array",
      "items": {
        "type": "string",
        "foo": "buzz"
      }
    }
  },
  "required": [
    "a_field",
    "some_fields"
  ]
}

With current master this outputs:

{
  "title": "Foo",
  "type": "object",
  "properties": {
    "a_field": {
      "title": "A Field",
      "foo": "buzz",
      "type": "string"
    },
    "some_fields": {
      "title": "Some Fields",
      "foo": "buzz",
      "type": "array",
      "items": {
        "type": "string",
        "foo": "buzz"
      }
    }
  },
  "required": [
    "a_field",
    "some_fields"
  ]
}

Diff:

 {
   "title": "Foo",
   "type": "object",
   "properties": {
     "a_field": {
       "title": "A Field",
-      "type": "string",
-      "foo": "buzz"
+      "foo": "buzz",
+      "type": "string"
     },
     "some_fields": {
       "title": "Some Fields",
+      "foo": "buzz",
       "type": "array",
       "items": {
         "type": "string",
         "foo": "buzz"
       }
     }
   },
   "required": [
     "a_field",
     "some_fields"
   ]
 }

Note the extra "foo" in some_fields - I think this is a regression?