Fixed extra_pip_arg parsing error in pip_repository rules.

[UebelAndre]

PR Checklist

Please check if your PR fulfills the following requirements:

• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

• Bugfix
• Feature (please, look at the "Scope of the project" section in the README.md file)
• Code style update (formatting, local variables)
• Refactoring (no functional changes, no api changes)
• Build related changes
• CI related changes
• Documentation content changes
• Other... Please describe:

What is the current behavior?

I ran into an issue where pip_repository rules would fail whenever flags were added to requirements.txt (or requirements_lock). With the diff below, the following exception occurs when running bazel test //... in any workspace (here I just happen to have chosen an example at random).

diff --git a/examples/pip_repository_annotations/requirements.txt b/examples/pip_repository_annotations/requirements.txt
index 51d1dfc..db762cb 100644
--- a/examples/pip_repository_annotations/requirements.txt
+++ b/examples/pip_repository_annotations/requirements.txt
@@ -4,6 +4,8 @@
 #
 #    bazel run //:requirements.update
 #
+--extra-index-url https://pypi.python.org/simple/
+
 wheel==0.37.1 \
     --hash=sha256:4bdcd7d840138086126cd09254dc6195fb4fc6f01c050a1d7236f2630db1d22a \
     --hash=sha256:e9a504e793efbca1b8e0e9cb979a249cf4a0a7b5b8c9e8b65a5e39d49529c1c4

ERROR: no such package '@pypi//': rules_python failed:  (Traceback (most recent call last):
  File "/usr/local/Cellar/python@3.9/3.9.9/Frameworks/Python.framework/Versions/3.9/lib/python3.9/runpy.py", line 197, in _run_module_as_main
    return _run_code(code, main_globals, None,
  File "/usr/local/Cellar/python@3.9/3.9.9/Frameworks/Python.framework/Versions/3.9/lib/python3.9/runpy.py", line 87, in _run_code
    exec(code, run_globals)
  File "/private/var/tmp/_bazel_user/1364d13d3d56e9f951a1c2e7037fc439/external/rules_python/python/pip_install/parse_requirements_to_bzl/__main__.py", line 5, in <module>
    main()
  File "/private/var/tmp/_bazel_user/1364d13d3d56e9f951a1c2e7037fc439/external/rules_python/python/pip_install/parse_requirements_to_bzl/__init__.py", line 213, in main
    install_requirements = parse_install_requirements(
  File "/private/var/tmp/_bazel_user/1364d13d3d56e9f951a1c2e7037fc439/external/rules_python/python/pip_install/parse_requirements_to_bzl/__init__.py", line 43, in parse_install_requirements
    extra_pip_args.extend(shlex.split(line))
AttributeError: 'str' object has no attribute 'extend'
)

Issue Number: N/A

What is the new behavior?

This fixes this exception by correcting the regression introduced in #589 and adds regression tests.

Does this PR introduce a breaking change?

• Yes
• No

Other information

[UebelAndre]

This matches the expectations of the function parameters before the regression:
https://github.com/bazelbuild/rules_python/blob/0.6.0/python/pip_install/parse_requirements_to_bzl/__init__.py#L68-L76