Fix data_structures to pass our Travis CI pytests (#1088)

* Fix data_structures to pass pytests

* Restore data_structures/stacks/__init__.py

Author: cclauss

Diff for: .travis.yml

@@ -11,8 +11,6 @@ script:
   - scripts/validate_filenames.py  # no uppercase and no spaces
   - mypy --ignore-missing-imports .
   - pytest . --doctest-modules
-      --ignore=data_structures/stacks/balanced_parentheses.py
-      --ignore=data_structures/stacks/infix_to_postfix_conversion.py
       --ignore=file_transfer_protocol/ftp_send_receive.py
       --ignore=file_transfer_protocol/ftp_client_server.py
       --ignore=machine_learning/linear_regression.py

Diff for: data_structures/stacks/balanced_parentheses.py

@@ -1,6 +1,7 @@
 from __future__ import print_function
 from __future__ import absolute_import
-from stack import Stack
+
+from .stack import Stack
 
 __author__ = 'Omkar Pathak'
 

Diff for: data_structures/stacks/infix_to_postfix_conversion.py

@@ -2,7 +2,7 @@
 from __future__ import absolute_import
 import string
 
-from stack import Stack
+from .stack import Stack
 
 __author__ = 'Omkar Pathak'