##### SUMMARY
acme_certificate fails on python 2.7
```
  File "/tmp/ansible_acme_certificate_payload_E_vyop/ansible_acme_certificate_payload.zip/ansible/modules/crypto/acme/acme_certificate.py", line 762, in f
AttributeError: 'list' object has no attribute 'clear'
```

##### ISSUE TYPE
- Bug Report

##### COMPONENT NAME
acme_certificate

##### ANSIBLE VERSION
```
ansible 2.9.0
```

##### OS / ENVIRONMENT
Distributor ID: Ubuntu
Description: Ubuntu 18.04.3 LTS
Release: 18.04
Codename: bionic
Python 2.7.15+

##### STEPS TO REPRODUCE
Issue a certificate with acme_certificate.

```yaml
  - name: "Request certificate for {{ CERT.CN }}"
    acme_certificate:
      account_key: /etc/ssl/private/account.key
      csr: "/etc/ssl/csr/{{ CERT.CN }}.csr"
      dest: "/etc/ssl/certs/{{ CERT.CN }}.crt"
      acme_directory: "https://acme-v01.api.letsencrypt.org/directory"
      agreement: "https://letsencrypt.org/documents/LE-SA-v1.2-November-15-2017.pdf"
      data: "{{ letsencrypt_challenge }}"
      challenge: "dns-01"
      force: "{{ csr.changed }}"
      remaining_days: "10"
```

##### EXPECTED RESULTS
Successful issued certificate.


##### ACTUAL RESULTS
```paste below
The full traceback is:
Traceback (most recent call last):
  File "/root/.ansible/tmp/ansible-tmp-1573040538.74-14433469763146/AnsiballZ_acme_certificate.py", line 102, in <module>
    _ansiballz_main()
  File "/root/.ansible/tmp/ansible-tmp-1573040538.74-14433469763146/AnsiballZ_acme_certificate.py", line 94, in _ansiballz_main
    invoke_module(zipped_mod, temp_path, ANSIBALLZ_PARAMS)
  File "/root/.ansible/tmp/ansible-tmp-1573040538.74-14433469763146/AnsiballZ_acme_certificate.py", line 40, in invoke_module
    runpy.run_module(mod_name='ansible.modules.crypto.acme.acme_certificate', init_globals=None, run_name='__main__', alter_sys=False)
  File "/usr/lib/python2.7/runpy.py", line 192, in run_module
    fname, loader, pkg_name)
  File "/usr/lib/python2.7/runpy.py", line 72, in _run_code
    exec code in run_globals
  File "/tmp/ansible_acme_certificate_payload_uUsYVr/ansible_acme_certificate_payload.zip/ansible/modules/crypto/acme/acme_certificate.py", line 1090, in <module>
  File "/tmp/ansible_acme_certificate_payload_uUsYVr/ansible_acme_certificate_payload.zip/ansible/modules/crypto/acme/acme_certificate.py", line 1061, in main
  File "/tmp/ansible_acme_certificate_payload_uUsYVr/ansible_acme_certificate_payload.zip/ansible/modules/crypto/acme/acme_certificate.py", line 921, in get_certificate
  File "/tmp/ansible_acme_certificate_payload_uUsYVr/ansible_acme_certificate_payload.zip/ansible/modules/crypto/acme/acme_certificate.py", line 766, in _new_cert_v1
  File "/tmp/ansible_acme_certificate_payload_uUsYVr/ansible_acme_certificate_payload.zip/ansible/module_utils/acme.py", line 943, in process_links
  File "/tmp/ansible_acme_certificate_payload_uUsYVr/ansible_acme_certificate_payload.zip/ansible/modules/crypto/acme/acme_certificate.py", line 762, in f
AttributeError: 'list' object has no attribute 'clear'
```
- "Deactivate authentication objects (authz) after issuing a certificate,
         or when issuing the certificate failed."
      - "Authentication objects are bound to an account key and remain valid
         for a certain amount of time, and can be used to issue certificates
         without having to re-authenticate the domain. This can be a security
         concern."
    type: bool
    default: no
    version_added: 2.6
  force:
    description:
      - Enforces the execution of the challenge and validation, even if an
        existing certificate is still valid for more than C(remaining_days).
      - This is especially helpful when having an updated CSR e.g. with
        additional domains for which a new certificate is desired.
    type: bool
    default: no
    version_added: 2.6
  retrieve_all_alternates:
    description:
      - "When set to C(yes), will retrieve all alternate trust chains offered by the ACME CA.
         These will not be written to disk, but will be returned together with the main
         chain as C(all_chains). See the documentation for the C(all_chains) return
         value for details."
    type: bool
    default: no
    version_added: "2.9"
  select_chain:
    description:
      - "Allows to specify criteria by which an (alternate) trust chain can be selected."