Accurate as of commit 868d49eccdd6582e4ee992f5f28a3e65ca5ebaa5

https://github.com/nmigen/nmigen/issues/407

It looks like a recent change to nmigen dependencies is causing ModuleNotFoundError's

```
Traceback (most recent call last):
  File "/home/colepoirier/src/nmigen/nmigen/_yosys.py", line 17, in <module>
    importlib_resources.files # py3.9+ stdlib
AttributeError: module 'importlib.resources' has no attribute 'files'

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/home/colepoirier/src/nmigen/nmigen/_yosys.py", line 19, in <module>
    import importlib_resources # py3.8- shim
ModuleNotFoundError: No module named 'importlib_resources'

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "decoder/pseudo/pywriter.py", line 8, in <module>
    from soc.decoder.power_pseudo import convert_to_python
  File "/home/colepoirier/src/soc/src/soc/decoder/power_pseudo.py", line 18, in <module>
    from soc.decoder.power_decoder import create_pdecode
  File "/home/colepoirier/src/soc/src/soc/decoder/power_decoder.py", line 84, in <module>
    from nmigen.cli import rtlil
  File "/home/colepoirier/src/nmigen/nmigen/cli.py", line 4, in <module>
    from .back import rtlil, cxxrtl, verilog, pysim
  File "/home/colepoirier/src/nmigen/nmigen/back/cxxrtl.py", line 1, in <module>
    from .._yosys import *
  File "/home/colepoirier/src/nmigen/nmigen/_yosys.py", line 21, in <module>
    import importlib_resources # py3.6- shim
ModuleNotFoundError: No module named 'importlib_resources'
```