Clean up set and file usage in get_all_imports

- Move logic that doesn't need to be inside of file context managers
  outside
- Remove redundant `set` and `list` function calls

pipreqs/pipreqs.py

@@ -114,13 +114,14 @@ def get_all_imports(
         cleaned_name, _, _ = name.partition('.')
         imports.add(cleaned_name)
 
-    packages = set(imports) - set(set(candidates) & set(imports))
+    packages = imports - (set(candidates) & imports)
     logging.debug('Found packages: {0}'.format(packages))
 
     with open(join("stdlib"), "r") as f:
-        data = [x.strip() for x in f.readlines()]
-        data = [x for x in data if x not in py2_exclude] if py2 else data
-        return list(set(packages) - set(data))
+        data = {x.strip() for x in f}
+
+    data = {x for x in data if x not in py2_exclude} if py2 else data
+    return list(packages - data)
 
 
 def filter_line(l):