text: Title: Python version in setup.py is outdated
  Body: **Describe the bug**
  
  #140 suggests that the projects supports Python 3.8.1, however the Python version specifier is `~3.6` which is equivalent of `>=3.6.0 <3.7.0`.
  
  **To Reproduce**
  
  When trying to install the package with poetry (`poetry add syrupy`) I get:
  
  ```
  [...]
  
  The current project's Python requirement (>=3.6) is not compatible with some of the required packages Python requirement:
    - syrupy requires Python ~=3.6
  
  Because no versions of syrupy match >0.4.2,<0.5.0
   and syrupy (0.4.2) requires Python ~=3.6, syrupy is forbidden.
  
  [...]
  ```
  
  As shown in the error message in `pyproject.toml` I have `python = ">=3.6"`.
  
  **Expected behavior**
  
  The package should install since `syrupy` supports all Python versions over 3.6
  