GA GoogleAnalytics installation error

[etj]

While installing GA extension we get this error:

(default) ckan@fao-clh-ckan-review:~/src/ckanext-googleanalytics$ pip install -r  requirements.txt
DEPRECATION: Python 2.7 reached the end of its life on January 1st, 2020. Please upgrade your Python as Python 2.7 is no longer maintained. pip 21.0 will drop support for Python 2.7 in January 2021. More details about Python 2 support in pip can be found at https://pip.pypa.io/en/latest/development/release-process/#python-2-support pip 21.0 will remove support for this functionality.
Requirement already satisfied: gdata>=2.0.0 in /usr/lib/ckan/default/lib/python2.7/site-packages (from -r requirements.txt (line 1)) (2.0.18)
Collecting google-api-python-client<1.7.0,>=1.6.1
  Using cached google_api_python_client-1.6.7-py2.py3-none-any.whl (56 kB)
Requirement already satisfied: pyOpenSSL>=16.2.0 in /usr/lib/ckan/default/lib/python2.7/site-packages (from -r requirements.txt (line 3)) (19.1.0)
Collecting httplib2<1dev,>=0.9.2
  Using cached httplib2-0.18.1.tar.gz (262 kB)
  Installing build dependencies ... done
  Getting requirements to build wheel ... done
ERROR: Exception:
Traceback (most recent call last):
  File "/usr/lib/ckan/default/local/lib/python2.7/site-packages/pip/_internal/cli/base_command.py", line 228, in _main
    status = self.run(options, args)
  File "/usr/lib/ckan/default/local/lib/python2.7/site-packages/pip/_internal/cli/req_command.py", line 182, in wrapper
    return func(self, options, args)
  File "/usr/lib/ckan/default/local/lib/python2.7/site-packages/pip/_internal/commands/install.py", line 324, in run
    reqs, check_supported_wheels=not options.target_dir
  File "/usr/lib/ckan/default/local/lib/python2.7/site-packages/pip/_internal/resolution/legacy/resolver.py", line 183, in resolve
    discovered_reqs.extend(self._resolve_one(requirement_set, req))
  File "/usr/lib/ckan/default/local/lib/python2.7/site-packages/pip/_internal/resolution/legacy/resolver.py", line 388, in _resolve_one
    abstract_dist = self._get_abstract_dist_for(req_to_install)
  File "/usr/lib/ckan/default/local/lib/python2.7/site-packages/pip/_internal/resolution/legacy/resolver.py", line 340, in _get_abstract_dist_for
    abstract_dist = self.preparer.prepare_linked_requirement(req)
  File "/usr/lib/ckan/default/local/lib/python2.7/site-packages/pip/_internal/operations/prepare.py", line 483, in prepare_linked_requirement
    req, self.req_tracker, self.finder, self.build_isolation,
  File "/usr/lib/ckan/default/local/lib/python2.7/site-packages/pip/_internal/operations/prepare.py", line 91, in _get_prepared_distribution
    abstract_dist.prepare_distribution_metadata(finder, build_isolation)
  File "/usr/lib/ckan/default/local/lib/python2.7/site-packages/pip/_internal/distributions/sdist.py", line 38, in prepare_distribution_metadata
    self._setup_isolation(finder)
  File "/usr/lib/ckan/default/local/lib/python2.7/site-packages/pip/_internal/distributions/sdist.py", line 96, in _setup_isolation
    reqs = backend.get_requires_for_build_wheel()
  File "/usr/lib/ckan/default/local/lib/python2.7/site-packages/pip/_vendor/pep517/wrappers.py", line 161, in get_requires_for_build_wheel
    'config_settings': config_settings
  File "/usr/lib/ckan/default/local/lib/python2.7/site-packages/pip/_vendor/pep517/wrappers.py", line 265, in _call_hook
    raise BackendUnavailable(data.get('traceback', ''))
BackendUnavailable

`requirements.txt' contains these deps:

gdata>=2.0.0
google-api-python-client>=1.6.1, <1.7.0
pyOpenSSL>=16.2.0

gdata and pyOpenSSL are properly installed, when tried one by one, while google-api-python-client is required in a rather old version which triggers the installation error.
The old (deprecated) version of google-api-python-client is required becouase of the python version.