Docs: pip dependencies for testing (#2728)

docs/contribute/pull_request.rst

@@ -52,6 +52,12 @@ C++
 
 Python
 ^^^^^^
+Necessary dependencies:
+
+.. code:: bash
+
+  pip install --user nose Cython
+
 If you want to run all tests:
 
 .. code:: bash
@@ -72,4 +78,4 @@ If you want to run a single test:
   export PYTHONPATH=python:topi/python
   rm -rf python/tvm/*.pyc python/tvm/*/*.pyc python/tvm/*/*/*.pyc
 
-  TVM_FFI=ctypes python -m nose -v tests/python/unittest/test_pass_storage_rewrite.py
\ No newline at end of file
+  TVM_FFI=ctypes python -m nose -v tests/python/unittest/test_pass_storage_rewrite.py