external/pip_requirements.txt: add six

This is an attempt to fix the following build error:

```
Traceback (most recent call last):
  File "Firestore/Protos/tmphmjWeu.py", line 25, in <module>
    import nanopb_generator as nanopb
  File "nanopb/generator/nanopb_generator.py", line 25, in <module>
    import google.protobuf.text_format as text_format
  File "protobuf/python/google/protobuf/text_format.py", line 51, in <module>
    import six
ImportError: No module named six
```

e.g. https://github.com/firebase/firebase-cpp-sdk/runs/6034310890

Author: dconeybe

external/pip_requirements.txt

@@ -1,2 +1,3 @@
 absl-py
 protobuf
+six