payload_utils: Add requirements.txt.

- This will be needed to run checker consistently
in different environments (e.g. the python used
by default by Recipes / PRESUBMIT.py doesn't have
protobuf).

- Also add common venv name (".venv") to gitignore.

- Steps to create:
  1. Create empty venv: python3 -m venv .venv
  2. Install protobuf: pip install protobuf
  3. Check unittests: python3 -m unittest discover -p *test.py
  4. Write requirements.txt: pip freeze > requirements.txt

BUG=chromium:1051187
TEST=Create venv, install requirements.txt, run unittests.

Change-Id: I81b2e15decd64a99535fb85cdfec0338290fe86f
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/config/+/2063050
Tested-by: Andrew Lamb <andrewlamb@chromium.org>
Auto-Submit: Andrew Lamb <andrewlamb@chromium.org>
Commit-Queue: Chris McDonald <cjmcdonald@chromium.org>
Reviewed-by: Chris McDonald <cjmcdonald@chromium.org>
diff --git a/.gitignore b/.gitignore
index 9acd0c1..ceed536 100644
--- a/.gitignore
+++ b/.gitignore
@@ -2,6 +2,7 @@
 util/bindings/descpb.bin
 *_pb2.py
 *.pyc
+.venv
 
 # Ignore nested protocol buffer repos but include
 # direct protocol buffer definition directories.
diff --git a/requirements.txt b/requirements.txt
new file mode 100644
index 0000000..ac86da8
--- /dev/null
+++ b/requirements.txt
@@ -0,0 +1,3 @@
+pkg-resources==0.0.0
+protobuf==3.11.3
+six==1.14.0