Add a fallback mechanism for missing dependencies

### Added

- Added support for Python 2.7.

- Added a fallback mechanism (opt-in) for missing dependencies.

### Changes

It will automatically find a suitable version constraint.

!!!warning

    `poetry` uses the PyPI JSON API to retrieve package information.

    However, some packages (like `boto3` for example) have missing dependency

    information due to bad packaging/publishing which means that `poetry` won't

    be able to properly resolve dependencies.

    To workaround it you can set the missing dependencies yourself in your `pyproject.toml`

    or you can tell `poetry` to use a fallback mechanism by setting the

    `settings.pypi.fallback` setting to `true`.

    ```bash

    poetry config settings.pypi.fallback true

    ```

    Note that this is temporary and should be avoided as much as possible since

    it increases the dependency resolution time drastically (up to 30 minutes in some cases).

    Any case of missing dependencies should be reported to https://github.com/sdispater/poetry/issues

### Version constraints

In our example, we are requesting the `pendulum` package with the version constraint `^1.4`.

    def content(self):

        return self._content

    def setting(self, setting_name):  # type: (str) -> Any

    def setting(self, setting_name, default=None):  # type: (str) -> Any

        """

        Retrieve a setting value.

        """

        config = self._raw_content

        for key in keys:

            if key not in config:

                return None

                return default

            config = config[key]

    def handle(self):

        # Load poetry and display errors, if any

        self.poetry.check(self.poetry.config, strict=True)

        self.poetry.check(self.poetry.local_config, strict=True)

        self.info('All set!')

        # Create config file if it does not exist

        if not self._config.file.exists():

            self._config.file.parent.mkdir(parents=True, exist_ok=True)

            self._config.file.write_text(TEMPLATE)

            with self._config.file.open() as f:

                f.write(TEMPLATE)

        if not self._auth_config.file.exists():

            self._auth_config.file.parent.mkdir(parents=True, exist_ok=True)

            self._auth_config.file.write_text(AUTH_TEMPLATE)

            with self._auth_config.file.open() as f:

                f.write(AUTH_TEMPLATE)

    def handle(self):

        if self.option('list'):

        boolean_normalizer = lambda val: True if val in ['true', '1'] else False

        unique_config_values = {

            'settings.virtualenvs.create': (boolean_validator, boolean_normalizer)

            'settings.virtualenvs.create': (boolean_validator, boolean_normalizer),

            'settings.pypi.fallback': (boolean_validator, boolean_normalizer),

        }

        if setting_key in unique_config_values:

        orig_k = k

        for key, value in contents.items():

            if k is None and key not in ['config', 'repositories']:

            if k is None and key not in ['config', 'repositories', 'settings']:

                continue

            if isinstance(value, dict) or key == 'repositories' and k is None: