Added a yapf file and formated the scripts and pytest code (#2675)

Summary:
Currently there was no formater in the scripts folder. I have added the yapf
configuration file and formated the python folders.

YAPF is a code formatter mainainted and developed at Google:
https://github.com/google/yapf
and I've set it up to use the codeformat they use there, but open to other options.

Test Plan:
```
yapf -i scripts/*.py
git status
```

.style.yapf

@@ -0,0 +1,4 @@
+[style]
+based_on_style = google
+indent_width = 4
+column_limit = 80

pytest/lib/branches.py

@@ -1,11 +1,11 @@
-
 import os
 import subprocess
 
 
 def current_branch():
     return os.environ.get('BUILDKITE_BRANCH') or subprocess.check_output([
-        "git", "rev-parse", "--symbolic-full-name", "--abbrev-ref", "HEAD"]).strip().decode()
+        "git", "rev-parse", "--symbolic-full-name", "--abbrev-ref", "HEAD"
+    ]).strip().decode()
 
 
 def compile_binary(branch):
@@ -46,19 +46,25 @@ def compile_current():
 
 def download_binary(branch):
     url = f'https://s3-us-west-1.amazonaws.com/build.nearprotocol.com/nearcore/Linux/{branch}/near'
-    subprocess.check_output(['curl', '--proto', '=https', '--tlsv1.2',
-                             '-sSfL', url, '-o', f'../target/debug/near-{branch}'])
+    subprocess.check_output([
+        'curl', '--proto', '=https', '--tlsv1.2', '-sSfL', url, '-o',
+        f'../target/debug/near-{branch}'
+    ])
     subprocess.check_output(['chmod', '+x', f'../target/debug/near-{branch}'])
     url = f'https://s3-us-west-1.amazonaws.com/build.nearprotocol.com/nearcore/Linux/{branch}/state-viewer'
-    subprocess.check_output(['curl', '--proto', '=https', '--tlsv1.2',
-                             '-sSfL', url, '-o', f'../target/debug/state-viewer-{branch}'])
+    subprocess.check_output([
+        'curl', '--proto', '=https', '--tlsv1.2', '-sSfL', url, '-o',
+        f'../target/debug/state-viewer-{branch}'
+    ])
     subprocess.check_output(
         ['chmod', '+x', f'../target/debug/state-viewer-{branch}'])
 
 
 def prepare_ab_test(other_branch):
     compile_current()
-    if os.environ.get('BUILDKITE') and other_branch in ['master', 'beta', 'stable']:
+    if os.environ.get('BUILDKITE') and other_branch in [
+            'master', 'beta', 'stable'
+    ]:
         download_binary(other_branch)
     else:
         compile_binary(other_branch)