text: Title: "ImportError: cannot import name 'json'" when debugging with pydevd
  Body: Hi!
  I'm receiving this error when attempting to debug with a pydevd -debugger.
  
  ```
  Traceback (most recent call last):
    File "C:\Program Files\JetBrains\PyCharm 2018.1.3\helpers\pydev\pydevd.py", line 1664, in <module>
      main()
    File "C:\Program Files\JetBrains\PyCharm 2018.1.3\helpers\pydev\pydevd.py", line 1658, in main
      globals = debugger.run(setup['file'], None, None, is_module)
    File "C:\Program Files\JetBrains\PyCharm 2018.1.3\helpers\pydev\pydevd.py", line 1068, in run
      pydev_imports.execfile(file, globals, locals)  # execute the script
    File "C:\Program Files\JetBrains\PyCharm 2018.1.3\helpers\pydev\_pydev_imps\_pydev_execfile.py", line 18, in execfile
      exec(compile(contents+"\n", file, 'exec'), glob, loc)
    File "C:\Program Files\JetBrains\PyCharm 2018.1.3\helpers\pycharm\_jb_pytest_runner.py", line 31, in <module>
      pytest.main(args, plugins_to_load)
    File "D:\Projects\Python\FlaskSQLAlchemyMasterClass\lib\site-packages\_pytest\config.py", line 54, in main
      config = _prepareconfig(args, plugins)
    File "D:\Projects\Python\FlaskSQLAlchemyMasterClass\lib\site-packages\_pytest\config.py", line 167, in _prepareconfig
      pluginmanager=pluginmanager, args=args
    File "D:\Projects\Python\FlaskSQLAlchemyMasterClass\lib\site-packages\pluggy\__init__.py", line 617, in __call__
      return self._hookexec(self, self._nonwrappers + self._wrappers, kwargs)
    File "D:\Projects\Python\FlaskSQLAlchemyMasterClass\lib\site-packages\pluggy\__init__.py", line 222, in _hookexec
      return self._inner_hookexec(hook, methods, kwargs)
    File "D:\Projects\Python\FlaskSQLAlchemyMasterClass\lib\site-packages\pluggy\__init__.py", line 216, in <lambda>
      firstresult=hook.spec_opts.get('firstresult'),
    File "D:\Projects\Python\FlaskSQLAlchemyMasterClass\lib\site-packages\pluggy\callers.py", line 196, in _multicall
      gen.send(outcome)
    File "D:\Projects\Python\FlaskSQLAlchemyMasterClass\lib\site-packages\_pytest\helpconfig.py", line 89, in pytest_cmdline_parse
      config = outcome.get_result()
    File "D:\Projects\Python\FlaskSQLAlchemyMasterClass\lib\site-packages\pluggy\callers.py", line 76, in get_result
      raise ex[1].with_traceback(ex[2])
    File "D:\Projects\Python\FlaskSQLAlchemyMasterClass\lib\site-packages\pluggy\callers.py", line 180, in _multicall
      res = hook_impl.function(*args)
    File "D:\Projects\Python\FlaskSQLAlchemyMasterClass\lib\site-packages\_pytest\config.py", line 981, in pytest_cmdline_parse
      self.parse(args)
    File "D:\Projects\Python\FlaskSQLAlchemyMasterClass\lib\site-packages\_pytest\config.py", line 1146, in parse
      self._preparse(args, addopts=addopts)
    File "D:\Projects\Python\FlaskSQLAlchemyMasterClass\lib\site-packages\_pytest\config.py", line 1098, in _preparse
      self.pluginmanager.load_setuptools_entrypoints("pytest11")
    File "D:\Projects\Python\FlaskSQLAlchemyMasterClass\lib\site-packages\pluggy\__init__.py", line 397, in load_setuptools_entrypoints
      plugin = ep.load()
    File "D:\Projects\Python\FlaskSQLAlchemyMasterClass\lib\site-packages\pkg_resources\__init__.py", line 2318, in load
      return self.resolve()
    File "D:\Projects\Python\FlaskSQLAlchemyMasterClass\lib\site-packages\pkg_resources\__init__.py", line 2324, in resolve
      module = __import__(self.module_name, fromlist=['__name__'], level=0)
    File "D:\Projects\Python\FlaskSQLAlchemyMasterClass\lib\site-packages\_pytest\assertion\rewrite.py", line 216, in load_module
      py.builtin.exec_(co, mod.__dict__)
    File "D:\Projects\Python\FlaskSQLAlchemyMasterClass\lib\site-packages\pytest_flask\plugin.py", line 11, in <module>
      from flask import json
  ImportError: cannot import name 'json'
  ```