Vendored version of cloudpickle does not support Python 3.8

[jonblum]

The version of cloudpickle currently vendored in srsly fails to import under Python 3.8 as a result of PEP 570-related changes to types.CodeType's signature:

  File "...../lib/python3.8/site-packages/srsly/__init__.py", line 7, in <module>
    from ._pickle_api import pickle_dumps, pickle_loads
  File "...../lib/python3.8/site-packages/srsly/_pickle_api.py", line 4, in <module>
    from . import cloudpickle
  File "...../lib/python3.8/site-packages/srsly/cloudpickle/__init__.py", line 1, in <module>
    from .cloudpickle import *
  File "...../lib/python3.8/site-packages/srsly/cloudpickle/cloudpickle.py", line 167, in <module>
    _cell_set_template_code = _make_cell_set_template_code()
  File "...../lib/python3.8/site-packages/srsly/cloudpickle/cloudpickle.py", line 148, in _make_cell_set_template_code
    return types.CodeType(
TypeError: an integer is required (got type bytes)

See also:
cloudpipe/cloudpickle#266
cloudpipe/cloudpickle#267

Updating to the latest version of cloudpickle should resolve this.