find_distributions breaks under python 3.8

This seems to be a regression, as 0.20 works fine.

Python 3.8.0b4 (default, Sep  2 2019, 20:50:55) 
[GCC 7.4.0] on linux
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>>> import importlib_metadata
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/matejcik/projekty/prace/trezor-firmware/python/.tox/py38/lib/python3.8/site-packages/importlib_metadata/__init__.py", line 527, in <module>
    __version__ = version(__name__)
  File "/home/matejcik/projekty/prace/trezor-firmware/python/.tox/py38/lib/python3.8/site-packages/importlib_metadata/__init__.py", line 494, in version
    return distribution(package).version
  File "/home/matejcik/projekty/prace/trezor-firmware/python/.tox/py38/lib/python3.8/site-packages/importlib_metadata/__init__.py", line 467, in distribution
    return Distribution.from_name(package)
  File "/home/matejcik/projekty/prace/trezor-firmware/python/.tox/py38/lib/python3.8/site-packages/importlib_metadata/__init__.py", line 181, in from_name
    dists = resolver(DistributionFinder.Context(name=name))
  File "<frozen importlib._bootstrap_external>", line 1385, in find_distributions
  File "/home/matejcik/projekty/prace/trezor-firmware/python/.tox/py38/lib/python3.8/re.py", line 275, in escape
    pattern = str(pattern, 'latin1')
TypeError: decoding to str: need a bytes-like object, Context found