Make argparse dependency unconditional. (#2249)

The primary motivation is to avoid a branch, giving bugs one fewer place to hide. But, as a bonus, more people get a more bugfixed version of argparse. (To use the example from the argparse docs, people stuck on Python 3.2.3 can get bugfixes that made it into the stdlib only in 3.2.4.)

acme/setup.py

@@ -8,6 +8,7 @@ version = '0.13.0.dev0'
 
 # Please update tox.ini when modifying dependency version requirements
 install_requires = [
+    'argparse',
     # load_pem_private/public_key (>=0.6)
     # rsa_recover_prime_factors (>=0.8)
     'cryptography>=0.8',
@@ -30,8 +31,6 @@ install_requires = [
 # Keep in sync with conditional_requirements.py.
 if sys.version_info < (2, 7):
     install_requires.extend([
-        # only some distros recognize stdlib argparse as already satisfying
-        'argparse',
         'mock<1.1.0',
     ])
 else: