.arcconfig

tests: hopefully fix `test-doctest.py` on Windows and more

1. Shell syntax understood by `shell=True` depends on the platform.
Instead, pass `shell=False` and call `sh` explicitly to interpret
the command correctly.

2. Stop setting `HGRCPATH=/dev/null`, so the setting
`experimental.evolution=createmarkers` is set correctly.
The reason I set HGRCPATH to /dev/null previously is because of
misunderstanding where I thought the Python script had no HGRC to edit.
As it turns out, there is in fact a valid temporary HGRC pointed to by
HGRCPATH in this context so we don't seem to need this. /shrug

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