Pin pythonnet to 2.5.2 (or compatible) in setup.py (#53)

When testing the built WHL file, I noticed that pythonnet 3 is still installed despite updating requirements.txt. It turns out that we need to update `install_requires` in setup.py as well.

Author: blair-ni

setup.py

@@ -42,7 +42,7 @@ def read_contents(file_to_read):
     packages=find_packages('src'),
     package_dir={'': 'src'},
     include_package_data=True,
-    install_requires=['pythonnet', 'PyYAML'],
+    install_requires=['pythonnet~=2.5.2', 'PyYAML'],
     tests_require=['pytest', 'numpy'],
     classifiers=[
         "Development Status :: 5 - Production/Stable",