##### 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'
```
the necessary certificate has to be created and served.
      It is I(not) the responsibility of this module to perform these steps."
   - "For details on how to fulfill these challenges, you might have to read through
      L(the main ACME specification,https://tools.ietf.org/html/rfc8555#section-8)
      and the L(TLS-ALPN-01 specification,https://tools.ietf.org/html/draft-ietf-acme-tls-alpn-05#section-3).
      Also, consider the examples provided for this module."
   - "The module includes experimental support for IP identifiers according to
      the L(current ACME IP draft,https://tools.ietf.org/html/draft-ietf-acme-ip-05)."
notes:
   - "At least one of C(dest) and C(fullchain_dest) must be specified."
   - "This module includes basic account management functionality.
      If you want to have more control over your ACME account, use the M(acme_account)
      module and disable account management for this module using the C(modify_account)
      option."
   - "This module was called C(letsencrypt) before Ansible 2.6. The usage
      did not change."
seealso:
  - name: The Let's Encrypt documentation
    description: Documentation for the Let's Encrypt Certification Authority.
                 Provides useful information for example on rate limits.
    link: https://letsencrypt.org/docs/
  - name: Buypass Go SSL
    description: Documentation for the Buypass Certification Authority.
                 Provides useful information for example on rate limits.
    link: https://www.buypass.com/ssl/products/acme
  - name: Automatic Certificate Management Environment (ACME)
    description: The specification of the ACME protocol (RFC 8555).
    link: https://tools.ietf.org/html/rfc8555
  - name: ACME TLS ALPN Challenge Extension
    description: The current draft specification of the C(tls-alpn-01) challenge.