Currently the operator allows to set requirement as list that needs to be hard coded.
It would be nice if airflow can support reading from file directly (something similar to how operators read sql file)
airflow/operators/python.py
Airflow through plugins. You can use string_args though.
    .. seealso::
        For more information on how to use this operator, take a look at the guide:
        :ref:`howto/operator:PythonVirtualenvOperator`
    :param python_callable: A python function with no references to outside variables,
        defined with def, which will be run in a virtualenv
    :type python_callable: function
    :param requirements: A list of requirements as specified in a pip install command
    :type requirements: list[str]
    :param python_version: The Python version to run the virtualenv with. Note that
        both 2 and 2.7 are acceptable forms.
    :type python_version: Optional[Union[str, int, float]]
    :param use_dill: Whether to use dill to serialize
        the args and result (pickle is default). This allow more complex types
        but requires you to include dill in your requirements.
    :type use_dill: bool
    :param system_site_packages: Whether to include
        system_site_packages in your virtualenv.
        See virtualenv documentation for more information.
    :type system_site_packages: bool
    :param op_args: A list of positional arguments to pass to python_callable.
    :type op_args: list
    :param op_kwargs: A dict of keyword arguments to pass to python_callable.
    :type op_kwargs: dict
    :param string_args: Strings that are present in the global var virtualenv_string_args,
        available to python_callable at runtime as a list[str]. Note that args are split
        by newline.
    :type string_args: list[str]
    :param templates_dict: a dictionary where the values are templates that
        will get templated by the Airflow engine sometime between