upgrade to protobuf 4.21.5, add quiet mode, refactor code, add unit test

- update README.txt
- rename generated code directory
- refactor code:
    - extract methods
    - make code testable

README.md

@@ -13,18 +13,15 @@ Extract two-factor authentication (2FA, TFA) secret keys from export QR codes of
 
 ## Dependencies
 
-The protobuf package of Google for proto3 is required for running this script. protobuf >= 3.14 is recommended.
-
-    pip install protobuf
+    pip install -r requirements.txt
 
 Known to work with
 
-* Python 3.6.12 and protobuf 3.14.0
-* Python 3.8.5 and protobuf 3.14.0
+* Python 3.10.6, protobuf 4.21.5, qrcode 7.3.1, and pillow 9.2
 
 ### Optional
 
-For printing QR codes, the qrcode module is required
+For printing QR codes, the qrcode module is required, otherwise it can be omitted.
 
     pip install qrcode[pil]
 
@@ -33,9 +30,11 @@ For printing QR codes, the qrcode module is required
 The export QR code of "Google Authenticator" contains the URL `otpauth-migration://offline?data=...`.
 The data parameter is a base64 encoded proto3 message (Google Protocol Buffers).
 
-Command for regeneration of Python code from proto3 message definition file (only necessary in case of changes of the proto3 message definition):
+Command for regeneration of Python code from proto3 message definition file (only necessary in case of changes of the proto3 message definition or new protobuf versions):
 
-    protoc --python_out=generated_python google_auth.proto
+    protoc --python_out=protobuf_generated_python google_auth.proto
+
+The generated protobuf Python code was generated by protoc 21.5 (https://github.com/protocolbuffers/protobuf/releases/tag/v21.5).
 
 ## References
 
@@ -63,3 +62,13 @@ Install [devbox](https://github.com/jetpack-io/devbox), which is a wrapper for n
 ```
 devbox shell
 ```
+
+## Unit Tests
+
+There are basic unit tests, see `unittest_extract_otp_secret_keys.py`.
+
+Run unit tests:
+
+```
+python -m unittest
+```