Use importlib_metadata instead of importlib.metadata

Make it backward compatible with every supported Python version.
Could be a fix to #545.

pygal/__init__.py

@@ -28,7 +28,8 @@
 import sys
 import traceback
 import warnings
-from importlib.metadata import entry_points
+
+from importlib_metadata import entry_points
 
 from pygal import maps
 from pygal.config import Config

pygal/test/test_maps.py

@@ -18,7 +18,7 @@
 # along with pygal. If not, see <http://www.gnu.org/licenses/>.
 """Map plugins tests are imported here"""
 
-from importlib.metadata import entry_points
+from importlib_metadata import entry_points
 
 # Load plugins tests
 for entry in entry_points(group="pygal.test.test_maps"):

tox.ini

@@ -9,6 +9,7 @@ deps =
      lxml
      pyquery
      cairosvg
+     importlib_metadata
 
 setenv =
     COVERAGE_FILE=.cov-{envname}