pytest: Exclude build directory for tests discovery

Due to test modules being copied in subdirectories of the
build directory by setuptools, it makes pytest fail by raising
ImportPathMismatchError exceptions when invoked from root
directory of the module.

So ignore the build folder to discover tests.

pytest.ini

 [pytest]
 addopts = --doctest-modules -p no:pytest_swh_core
-norecursedirs = docs .*
+norecursedirs = build docs .*
 markers =
     fs: tests that involve filesystem ios
     requires_optional_deps: tests in test_cli.py that should not run if optional dependencies are not installed