Fix error on inline comments in Python dependencies (#708)

It appears that the distlib parsing function we're using here can't
handle inline comments and raises a syntax error. After digging into the
setuptools sources, it appears that these comments are dropped between
parsing the file and parsing the individual dependencies, so we can do
the same.

Author: cottsay

colcon_core/package_augmentation/python.py

@@ -121,6 +121,9 @@ def create_dependency_descriptor(requirement_string):
         '<': 'version_lt',
     }
 
+    # Drop inline comments
+    requirement_string = requirement_string.partition(' #')[0]
+
     requirement = parse_requirement(requirement_string)
     metadata = {
         'origin': 'python',

test/test_package_identification_python.py

@@ -161,3 +161,7 @@ def test_create_dependency_descriptor():
     dep = create_dependency_descriptor(multi_str)
     assert dep.metadata['version_gte'] == '2.2.0'
     assert dep.metadata['version_lte'] == '3.2.0'
+
+    commented_str = 'pkgname  # This is needed for foo'
+    dep = create_dependency_descriptor(commented_str)
+    assert dep.name == 'pkgname'