Import only Python 3 modules.

Set `universal_newlines` argument to True for `subprocess.Popen` calls
that require the `stdout_data` from `proc.communicate()` to be of type
`str`.

- Convert `b` to `bytes` if it is a `str`.
- Convert the base64 encoded `bytes` to `str` before using `replace`
  for stripping `=`.

Files opened using `tempfile.NamedTemporaryFile` are opened by default
in binary mode. Writing `str` to the temporary file causes it error
out.

Update error handling sections to treat the result of `HTTPError.read`
as `bytes`.

Decode `signed_der64` to `str` before passing it to `textwrap.wrap`.

Encode accountkey_json (to bytes) before passing to hashlib.sha256.

- Import urllib.request
- Remove python2 urllib2 import.

Set `universal_newlines` argument to True for the `subprocess.Popen`
that reads the public key so that the `out` returned from
proc.communicate is of type `str`.

Files opened using `tempfile.NamedTemporaryFile` are opened by default
in binary mode. Writing `str` to the temporary file causes it error
out.

- Convert `b` to `bytes` if it is a `str`.
- Convert the base64 encoded `bytes` to `str` before using `replace`
  for stripping `=`.

Initial version.

The ACME v2 of the sign_csr.py and revoke_crt.py require the user
account private key.

Update this section to reflect the ACME v2 version of the
`sign_csr.py` script.