setup.py: Exclude "tests" package

67b957d moved the tests from nmigen/test/ to tests/, and removed the
exclude= parameter from find_packages() in setup.py. However, even if
the new location is not inside the module tree, it is still found by
find_packages(), resulting in a stray "tests" module on the system.

setup.py

@@ -48,7 +48,7 @@ setup(
         "builtin-yosys": ["nmigen-yosys>=0.9.post3527.*"],
         "remote-build": ["paramiko~=2.7"],
     },
-    packages=find_packages(),
+    packages=find_packages(exclude=["tests*"]),
     entry_points={
         "console_scripts": [
             "nmigen-rpc = nmigen.rpc:main",