Include requirements.in in data_files for source distribution

Ensure the source requirements file is included in the package
distribution since setup.py now reads from it.

Co-authored-by: hsluoyz <3787410+hsluoyz@users.noreply.github.com>

setup.py

diff --git a/setup.py b/setup.py
index 22b311d..09566dc 100644
--- a/setup.py
+++ b/setup.py
@@ -47,7 +47,7 @@
     packages=find_packages(exclude=["docs", "test*"]),
     install_requires=install_requires,
     python_requires=">=3.6",
-    data_files=[desc_file, "requirements.txt"],
+    data_files=[desc_file, "requirements.in", "requirements.txt"],
     include_package_data=True,
     dependency_links=dependency_links,
     license="Apache 2.0",