Fix type hints of parse_obj and similar methods

[erosennin]

Change Summary

Methods like BaseModel.parse_obj() or BaseModel.copy() have their return type specified as BaseModel. Because of that, valid code like this:

from pydantic import BaseModel

class User(BaseModel):
    id: int
    name: str

u = User.parse_obj({
    'id': 11,
    'name': 'Acedia',
})

print(u.id, u.name)

does not type check:

example.py:12: error: "BaseModel" has no attribute "id"
example.py:12: error: "BaseModel" has no attribute "name"

This PR fixes that.

Related issue number

Checklist

• Unit tests for the changes exist
• Tests pass on CI and coverage remains at 100%
• Documentation reflects the changes where applicable
• HISTORY.rst has been updated
  • if this is the first change since a release, please add a new section
  • include the issue number or this pull request number #<number>
  • include your github username @<whomever>

[codecov]

Codecov Report

Merging #405 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff          @@
##           master   #405   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files          14     14           
  Lines        2068   2068           
  Branches      420    420           
=====================================
  Hits         2068   2068

[samuelcolvin]

otherwise LGTM.

[erosennin]

Done.

[samuelcolvin]

awesome. Thank you very much.

I'll fix another thing and deploy in a day or two.