AST Parser Matching Name assignments should choose last match

[techalchemy]

Currently, the setup.py AST parser will look for matches by name when matching assignments in setup.py. If a name has been assigned multiple times, however, the first assignment is chosen instead of the most recent assignment, which can cause major problems for the parser which will be working with an incomplete / unparsed object.

For example, when unparsing the following ast.BinOp instance:

	matches = entry['key'][0]['matches']
	matches = [x for x in matches if 'some_string' not in x]
	entry['key'][0]['matches'] = matches + other_list

the first assignment of matches is chosen, causing the following exception:

  File "<stdin>", line 1, in <module>
  File "/home/techalchemy/git/requirementslib/src/requirementslib/models/setup_info.py", line 1201, in ast_parse_setup_py
    ast_analyzer.unmap_binops()
  File "/home/techalchemy/git/requirementslib/src/requirementslib/models/setup_info.py", line 729, in unmap_binops
    self.binOps_map[binop] = ast_unparse(binop, analyzer=self)
  File "/home/techalchemy/git/requirementslib/src/requirementslib/models/setup_info.py", line 1017, in ast_unparse
    left_item = unparse(item.left)
  File "/home/techalchemy/git/requirementslib/src/requirementslib/models/setup_info.py", line 1040, in ast_unparse
    items = unparse(analyzer.assignments[assignment])
  File "/home/techalchemy/git/requirementslib/src/requirementslib/models/setup_info.py", line 1140, in ast_unparse
    val = unparse(item.value, recurse=False)
  File "/home/techalchemy/git/requirementslib/src/requirementslib/models/setup_info.py", line 1000, in ast_unparse
    unparsed = unparse(item.value)
  File "/home/techalchemy/git/requirementslib/src/requirementslib/models/setup_info.py", line 1000, in ast_unparse
    unparsed = unparse(item.value)
  File "/home/techalchemy/git/requirementslib/src/requirementslib/models/setup_info.py", line 1004, in ast_unparse
    unparsed = unparsed[unparse(item.slice.value)]
TypeError: string indices must be integers

string indices must be integers