Tests failures with packaging 26.0

In packaging release 26.0, they've changed the string representation of requirements with URL like:

```diff
-foo@ git+http://foo.com@ref#egg=foo
+foo @ git+http://foo.com@ref#egg=foo
```

See https://github.com/pypa/packaging/issues/935 and https://github.com/pypa/packaging/pull/953

Because of that change, some tests in this project fail:

```
=========================== short test summary info ============================
FAILED tests/pip_requirements_parser_tests/unit/test_req.py::TestInstallRequirement::test_pep440_url_link_requirement
FAILED tests/pip_requirements_parser_tests/unit/test_req.py::TestInstallRequirement::test_pep440_wheel_link_requirement
FAILED tests/pip_requirements_parser_tests/unit/test_req_file.py::TestProcessFile::test_parse_name_at_url_to_wheel_with_packaging
FAILED tests/pip_requirements_parser_tests/unit/test_req_file.py::TestProcessFile::test_parse_name_at_url__with_packaging
FAILED tests/pip_requirements_parser_tests/unit/test_req_file.py::TestProcessFile::test_parse_name_at_vcs_url_to_wheel_with_packaging
FAILED tests/pip_requirements_parser_tests/unit/test_req_file.py::TestProcessFile::test_can_dumps_name_at_url_to_wheel
FAILED tests/pip_requirements_parser_tests/unit/test_req_file.py::TestProcessFile::test_can_dumps_name_at_url_to_wheel_with_space
FAILED tests/test_pip_api_parse_requirements.py::test_parse_requirements_PEP508[test_5080]
FAILED tests/test_pip_api_parse_requirements.py::test_parse_requirements_PEP508[test_5081]
XFAIL tests/test_packaging_legacy_version.py::TestLegacyVersion::test_legacy_version_is_deprecated - We vendored packaging to reinstate LegacyVersion
===== 9 failed, 8105 passed, 5 skipped, 156 deselected, 1 xfailed in 4.80s =====
```