text: Title: pandera 0.12.0 does not support Python 3.7
  Body: When trying to import pandera 0.12.0 in python 3.7, I get the following error message:
  
  Traceback (most recent call last):
    File "<stdin>", line 1, in <module>
    File "...\lib\site-packages\pandera\__init__.py", line 50, in <module>
      from . import errors, pandas_accessor, typing
    File "...\lib\site-packages\pandera\pandas_accessor.py", line 7, in <module>
      from . import schemas
    File "...\lib\site-packages\pandera\schemas.py", line 13, in <module>
      from typing import (
  ImportError: cannot import name 'Literal' from 'typing' (...\lib\typing.py)
  
  A quick search shows that Literal does not exist in typing in python 3.7 (https://stackoverflow.com/questions/61206437/importerror-cannot-import-name-literal-from-typing)
  
  A clear and concise description of what the bug is.
  
  - [X] I have checked that this issue has not already been reported.
  - [X] I have confirmed this bug exists on the latest version of pandera.
  - [ ] (optional) I have confirmed this bug exists on the master branch of pandera.
  
  **Note**: Please read [this guide](https://matthewrocklin.com/blog/work/2018/02/28/minimal-bug-reports) detailing how to provide the necessary information for us to reproduce your bug.
  
  #### Code Sample, a copy-pastable example
  
  ```python
  import pandera
  
  ```
  
  #### Expected behavior
  A clear and concise description of what you expected to happen.
  
  No error.
  
  #### Desktop (please complete the following information):
  
   - OS: Windows 10
   - Browser [e.g. chrome, safari]  N/A
   - Version: pandera 0.12.0, python 3.7
  
  #### Screenshots
  If applicable, add screenshots to help explain your problem.
  
  #### Additional context
  Add any other context about the problem here.
  