CI: fix code coverage collection.

This has been broken since commit d71e19e2 (2020-07-22).

.coveragerc

@@ -4,7 +4,6 @@ include =
   nmigen/*
 omit =
   nmigen/test/*
-  */__init__.py
 
 [report]
 exclude_lines =

.github/workflows/main.yaml

@@ -33,7 +33,7 @@ jobs:
     - name: Run tests
       run: |
         export NMIGEN_USE_YOSYS=builtin YOSYS=yowasp-yosys SBY=yowasp-sby SMTBMC=yowasp-yosys-smtbmc
-        python -m unittest discover
+        python -m coverage run -m unittest
         codecov
   document:
     runs-on: ubuntu-latest

nmigen/__init__.py

@@ -8,7 +8,7 @@ except ImportError:
     # No importlib_metadata. This shouldn't normally happen, but some people prefer not installing
     # packages via pip at all, instead using PYTHONPATH directly or copying the package files into
     # `lib/pythonX.Y/site-packages`. Although not a recommended way, we still try to support it.
-    __version__ = "unknown"
+    __version__ = "unknown" # :nocov:
 
 
 from .hdl import *