Tidy up Python file handlers

Rather than using `f = open(path).read()`, which leaves the file open
for an indeterminate period of time, switch to the `with open(path) as f`
idiom, which ensures the file is always closed correctly.

utils/swift-bench.py

@@ -154,9 +154,8 @@ main()
 """
 
     benchRE = re.compile("^\s*func\s\s*bench_([a-zA-Z0-9_]+)\s*\(\s*\)\s*->\s*Int\s*({)?\s*$")
-    f = open(name)
-    lines = f.readlines()
-    f.close()
+    with open(name) as f:
+      lines = list(f)
     output = header
     lookingForCurlyBrace = False
     testNames = []
@@ -190,9 +189,8 @@ main()
       output += mainBody % (n, n)
     processedName = 'processed_' + os.path.basename(name)
     output += mainEnd
-    f = open(processedName, 'w')
-    f.write(output)
-    f.close()
+    with open(processedName, 'w') as f:
+      f.write(output)
     for n in testNames:
       self.tests[name+":"+n].processedSource = processedName
 
@@ -210,9 +208,8 @@ main()
 extern "C" int32_t opaqueGetInt32(int32_t x) { return x; }
 extern "C" int64_t opaqueGetInt64(int64_t x) { return x; }
 """
-    f = open('opaque.cpp', 'w')
-    f.write(fileBody)
-    f.close()
+    with open('opaque.cpp', 'w') as f:
+      f.write(fileBody)
     # TODO: Handle subprocess.CalledProcessError for this call:
     self.runCommand(['clang++', 'opaque.cpp', '-o', 'opaque.o', '-c', '-O2'])