Update fails on BackendUnavailable

Please, help - the next update fails. Many previous updates worked fine, but now:

Backing up sites…
/home/frappe/frappe-bench/env/local/lib/python2.7/site-packages/cryptography/hazmat/primitives/constant_time.py:26: CryptographyDeprecationWarning: Support for your Python version is deprecated. The next version of cryptography will remove support. Please upgrade to a 2.7.x release that supports hmac.compare_digest as soon as possible.
utils.DeprecatedIn23,
INFO:bench.app:pulling frappe
$ git pull upstream version-11
From GitHub - frappe/frappe: Low code web framework for real world applications, in Python and Javascript

  • branch version-11 → FETCH_HEAD
    Already up-to-date.
    $ find . -name “*.pyc” -delete
    INFO:bench.app:pulling erpnext
    $ git pull upstream version-11
    From GitHub - frappe/erpnext: Free and Open Source Enterprise Resource Planning (ERP)
  • branch version-11 → FETCH_HEAD
    Already up-to-date.
    $ find . -name “*.pyc” -delete
    INFO:bench.app:pulling erpnext_demo
    $ git pull upstream master
    From GitHub - frappe/erpnext_demo: Deprecated with Version 7 (will work till v6)
  • branch master → FETCH_HEAD
    Already up-to-date.
    $ find . -name “*.pyc” -delete
    Updating Python libraries…
    $ /home/frappe/frappe-bench/env/bin/pip install -q -U pip
    /home/frappe/frappe-bench/env/local/lib/python2.7/site-packages/cryptography/hazmat/primitives/constant_time.py:26: CryptographyDeprecationWarning: Support for your Python version is deprecated. The next version of cryptography will remove support. Please upgrade to a 2.7.x release that supports hmac.compare_digest as soon as possible.
    utils.DeprecatedIn23,
    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 Please upgrade your Python as Python 2.7 is no longer maintained.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
    INFO:bench.app:installing frappe
    $ /home/frappe/frappe-bench/env/bin/pip install -q -U -e /home/frappe/frappe-bench/apps/frappe
    /home/frappe/frappe-bench/env/local/lib/python2.7/site-packages/cryptography/hazmat/primitives/constant_time.py:26: CryptographyDeprecationWarning: Support for your Python version is deprecated. The next version of cryptography will remove support. Please upgrade to a 2.7.x release that supports hmac.compare_digest as soon as possible.
    utils.DeprecatedIn23,
    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
    ERROR: Exception:
    Traceback (most recent call last):
    File “/home/frappe/frappe-bench/env/local/lib/python2.7/site-packages/pip/_internal/cli/base_command.py”, line 188, in _main
    status = self.run(options, args)
    File “/home/frappe/frappe-bench/env/local/lib/python2.7/site-packages/pip/_internal/cli/req_command.py”, line 185, in wrapper
    return func(self, options, args)
    File “/home/frappe/frappe-bench/env/local/lib/python2.7/site-packages/pip/_internal/commands/install.py”, line 333, in run
    reqs, check_supported_wheels=not options.target_dir
    File “/home/frappe/frappe-bench/env/local/lib/python2.7/site-packages/pip/_internal/resolution/legacy/resolver.py”, line 179, in resolve
    discovered_reqs.extend(self._resolve_one(requirement_set, req))
    File “/home/frappe/frappe-bench/env/local/lib/python2.7/site-packages/pip/_internal/resolution/legacy/resolver.py”, line 362, in _resolve_one
    abstract_dist = self._get_abstract_dist_for(req_to_install)
    File “/home/frappe/frappe-bench/env/local/lib/python2.7/site-packages/pip/_internal/resolution/legacy/resolver.py”, line 314, in _get_abstract_dist_for
    abstract_dist = self.preparer.prepare_linked_requirement(req)
    File “/home/frappe/frappe-bench/env/local/lib/python2.7/site-packages/pip/_internal/operations/prepare.py”, line 488, in prepare_linked_requirement
    req, self.req_tracker, self.finder, self.build_isolation,
    File “/home/frappe/frappe-bench/env/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 “/home/frappe/frappe-bench/env/local/lib/python2.7/site-packages/pip/_internal/distributions/sdist.py”, line 38, in prepare_distribution_metadata
    self._setup_isolation(finder)
    File “/home/frappe/frappe-bench/env/local/lib/python2.7/site-packages/pip/_internal/distributions/sdist.py”, line 96, in _setup_isolation
    reqs = backend.get_requires_for_build_wheel()
    File “/home/frappe/frappe-bench/env/local/lib/python2.7/site-packages/pip/_vendor/pep517/wrappers.py”, line 161, in get_requires_for_build_wheel
    ‘config_settings’: config_settings
    File “/home/frappe/frappe-bench/env/local/lib/python2.7/site-packages/pip/_vendor/pep517/wrappers.py”, line 265, in _call_hook
    raise BackendUnavailable(data.get(‘traceback’, ‘’))
    BackendUnavailable

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