Generate constraints file to pin deps in Docker images

Dockerfiles pin versions using constraints file

Pulling out strip_hashes and add --no-deps flag

tools/dev_constraints.txt

@@ -1,4 +1,4 @@
-# Specifies Python package versions for development.
+# Specifies Python package versions for development and building Docker images.
 # It includes in particular packages not specified in letsencrypt-auto's requirements file.
 # Some dev package versions specified here may be overridden by higher level constraints
 # files during tests (eg. letsencrypt-auto-source/pieces/dependency-requirements.txt).

tools/pip_install.py

@@ -19,6 +19,7 @@ import tempfile
 
 import merge_requirements as merge_module
 import readlink
+import strip_hashes
 
 
 def find_tools_path():
@@ -47,10 +48,7 @@ def certbot_normal_processing(tools_path, test_constraints):
     with open(certbot_requirements, 'r') as fd:
         data = fd.readlines()
     with open(test_constraints, 'w') as fd:
-        for line in data:
-            search = re.search(r'^(\S*==\S*).*$', line)
-            if search:
-                fd.write('{0}{1}'.format(search.group(1), os.linesep))
+        fd.write(strip_hashes.main(data))
 
 
 def merge_requirements(tools_path, requirements, test_constraints, all_constraints):

tools/strip_hashes.py

@@ -0,0 +1,16 @@
+#!/usr/bin/env python
+
+import os
+import re
+import sys
+
+def main(args):
+    out_lines = []
+    for line in args:
+        search = re.search(r'^(\S*==\S*).*$', line)
+        if search:
+            out_lines.append(search.group(1))
+    return os.linesep.join(out_lines)
+
+if __name__ == '__main__':
+    print(main(sys.argv[1:]))