pyproject.toml not parseable by pip 18.1 (debian 10)

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# Issue description

failed to install pyzmq





# Environment



* libzmq version (commit hash if unreleased): 24.0.1

* OS:  Debian GNU/Linux 10 \n \l  （arm64）

Python 3.7.3

pip 18.1 from /usr/lib/python3/dist-packages/pip (python 3.7)



# Minimal test code / Steps to reproduce the issue



1.  pip3 install pyzmq





# What's the actual result? (include assertion message & call stack if applicable)



failed to install zmq

root@RK3568:/home/admin# pip3 install pyzmq

Collecting pyzmq

  Using cached https://files.pythonhosted.org/packages/46/0d/b06cf99a64d4187632f4ac9ddf6be99cd35de06fe72d75140496a8e0eef5/pyzmq-24.0.1.tar.gz

Exception:

Traceback (most recent call last):

  File "/usr/lib/python3/dist-packages/pip/_internal/cli/base_command.py", line 143, in main

    status = self.run(options, args)

  File "/usr/lib/python3/dist-packages/pip/_internal/commands/install.py", line 338, in run

    resolver.resolve(requirement_set)

  File "/usr/lib/python3/dist-packages/pip/_internal/resolve.py", line 102, in resolve

    self._resolve_one(requirement_set, req)

  File "/usr/lib/python3/dist-packages/pip/_internal/resolve.py", line 256, in _resolve_one

    abstract_dist = self._get_abstract_dist_for(req_to_install)

  File "/usr/lib/python3/dist-packages/pip/_internal/resolve.py", line 209, in _get_abstract_dist_for

    self.require_hashes

  File "/usr/lib/python3/dist-packages/pip/_internal/operations/prepare.py", line 298, in prepare_linked_requirement

    abstract_dist.prep_for_dist(finder, self.build_isolation)

  File "/usr/lib/python3/dist-packages/pip/_internal/operations/prepare.py", line 100, in prep_for_dist

    self.req.load_pyproject_toml()

  File "/usr/lib/python3/dist-packages/pip/_internal/req/req_install.py", line 428, in load_pyproject_toml

    str(self)

  File "/usr/lib/python3/dist-packages/pip/_internal/pyproject.py", line 43, in load_pyproject_toml

    pp_toml = pytoml.load(f)

  File "/usr/share/python-wheels/pytoml-0.1.2-py2.py3-none-any.whl/pytoml/parser.py", line 303, in load

    filename=fin.name)

  File "/usr/share/python-wheels/pytoml-0.1.2-py2.py3-none-any.whl/pytoml/parser.py", line 370, in loads

    toks.expect('=', 'expected_equals')

  File "/usr/share/python-wheels/pytoml-0.1.2-py2.py3-none-any.whl/pytoml/parser.py", line 250, in expect

    self.error(error_text)

  File "/usr/share/python-wheels/pytoml-0.1.2-py2.py3-none-any.whl/pytoml/parser.py", line 253, in error

    raise TomlError(message, self.pos[0][0], self.pos[0][1], self._filename)

pytoml.core.TomlError: /tmp/pip-install-n2sovrrt/pyzmq/pyproject.toml(60, 12): expected_equals





# What's the expected result?



should install success

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