Some failing tests on windows

Thanks for `pip-requirements-parser`!

I'm looking to [package this for `conda-forge`](https://github.com/conda-forge/staged-recipes/pull/18293), and while, as a noarch package, we'd eventually only be building releases on linux, the initial review is built on the Big Three operating systems. Not-entirely-surprisingly, there are some [gotchas on windows](https://dev.azure.com/conda-forge/feedstock-builds/_build/results?buildId=472249&view=logs&j=240f1fee-52bc-5498-a14a-8361bde76ba0&t=06d62fd2-ab49-5add-9ee5-83b6882d7d2b&l=2218):

```
FAILED pip_requirements_parser_tests/test_requirements_file.py::test_RequirementsFile_to_dict[C:\\bld\\pip-requirements-parser_1646957052592\\test_tmp\\tests\\pip_requirements_parser_tests\\data\\requirements\\packaging-docs-examples.txt]
FAILED pip_requirements_parser_tests/test_requirements_file.py::test_RequirementsFile_to_dict[C:\\bld\\pip-requirements-parser_1646957052592\\test_tmp\\tests\\pip_requirements_parser_tests\\data\\requirements\\pip-docs-examples-vcs.txt]
FAILED pip_requirements_parser_tests/test_requirements_file.py::test_RequirementsFile_to_dict[C:\\bld\\pip-requirements-parser_1646957052592\\test_tmp\\tests\\pip_requirements_parser_tests\\data\\requirements\\pip-docs-examples.txt]
FAILED pip_requirements_parser_tests/test_requirements_file.py::test_RequirementsFile_dumps_unparse[C:\\bld\\pip-requirements-parser_1646957052592\\test_tmp\\tests\\pip_requirements_parser_tests\\data\\requirements\\pip-docs-examples-vcs.txt]
FAILED pip_requirements_parser_tests/test_requirements_file.py::test_RequirementsFile_dumps_unparse[C:\\bld\\pip-requirements-parser_1646957052592\\test_tmp\\tests\\pip_requirements_parser_tests\\data\\requirements\\pip-docs-examples.txt]
FAILED pip_requirements_parser_tests/unit/test_req.py::TestInstallRequirement::test_unexisting_path
```

I intend to skip them and move on, as they _look_ like some test assumptions (maybe not even from this codebase), but figured it was worth documenting here.

Thanks again!