Formatting changes to hash-built-wheels script

Mostly nitpicks, geared toward making the script more Pythonic and
intuitive. Another pass is warranted to flesh out comments and
docstrings, we can squash during final review.

install_files/ansible-base/roles/build-securedrop-app-code-deb-pkg/files/hash_built_wheels.py

@@ -3,20 +3,19 @@
 import glob
 import hashlib
 import os
-import re
-import sys
 
 REQUIREMENTS_FILE = os.environ['SD_REQUIREMENTS']
 WHEELHOUSE = os.environ['SD_WHEELHOUSE']
 LOCK_FILE = os.environ['SD_PIP_LOCK']
 
 
 def main():
-    f = open(LOCK_FILE, "r")
-    lockfile = f.readlines()
-    f.close()
+
+    with open(LOCK_FILE, "r") as f:
+        lockfile = f.readlines()
+
     # remove newlines
-    lockfile = list(map(lambda x: x.strip(), lockfile))
+    lockfile = [x.strip() for x in lockfile]
     new_reqs = ""
 
     for line in lockfile:
@@ -25,9 +24,8 @@ def main():
         package_string = "{}-{}".format(package.replace("-", "_"), version)
         new_reqs += "{}=={}".format(package, version)
         new_reqs += " --hash=sha256:{}\n".format(get_hash_for_package(package_string))
-    reqs_file = open(REQUIREMENTS_FILE, "w")
-    reqs_file.write(new_reqs)
-    reqs_file.close()
+    with open(REQUIREMENTS_FILE, "w") as reqs_file:
+        reqs_file.write(new_reqs)
 
 
 def get_hash_for_package(package):
@@ -38,11 +36,10 @@ def get_hash_for_package(package):
 
 def get_wheel(name):
     result = glob.glob(name + "*.whl")
-    if(len(result) == 1):
-        return result[0]
-    else:
-        print("There are several wheels matching package name", file=sys.stderr) # noqa
-        sys.exit(1)
+    if len(result) != 1:
+        msg = "There are several wheels matching package {}".format(name)
+        raise Exception(msg)
+    return result[0]
 
 
 def sha256_file(path):