Apply some pyupgrade suggestions

io.open() is an alias for for the builtin open() function:
	https://docs.python.org/3/library/io.html#io.open

If open() cannot open a file, an OSError is raised:
	https://docs.python.org/3/library/functions.html#open

EnvironmentError is kept for compatibility with previous versions;
starting from Python 3.3, it is an alias of OSError:
	https://docs.python.org/3/library/exceptions.html#EnvironmentError

Directly yield from an iterable instead of iterating to yield items.

tests/test_spec_examples.py

@@ -13,7 +13,6 @@
 # You should have received a copy of the GNU General Public License
 # along with this program.  If not, see <http://www.gnu.org/licenses/>.
 
-from io import open
 import os
 
 from tests.common import RuleTestCase