Set lower bound version for install requirements (#155)

Co-authored-by: Harris.Chu <1726587+HarrisChu@users.noreply.github.com>
Co-authored-by: Wey Gu <weyl.gu@gmail.com>

setup.py

@@ -22,7 +22,12 @@
     long_description=long_description,
     long_description_content_type='text/markdown',
     url='https://github.com/vesoft-inc/nebula-python',
-    install_requires=['httplib2', 'future', 'six', 'pytz'],
+    install_requires=[
+        'httplib2 >= 0.20.0',
+        'future >= 0.18.0',
+        'six >= 1.16.0',
+        'pytz >= 2021.1',
+    ],
     packages=find_packages(),
     platforms=['3.6, 3.7'],
     package_dir={'nebula3': 'nebula3'},