Fix unclosed file warning

[ckleemann]

Description

Fix the unclosed file handle warning by using an context-manager for each open file.

Related Issue

#411

Motivation and Context

It fixes the unclosed file warning for each file analyzed.

How Has This Been Tested?

I let run the tox tests as well as the nosetests. Also I checked with a project of mine if the warning disappeared.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• My change requires a change to the dependencies
• I have updated the dependencies accordingly
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.