change event, push to pull_request (#348)

.github/workflows/deploy-test-pypi.yml

 name: deploy-test-pypi
 
-on: [push]
+on: [pull_request]
 
 jobs:
   build:

.github/workflows/docs.yml

 name: docs
 
-on: [push]
+on: [pull_request]
 
 jobs:
   build:

.github/workflows/flake8.yml

 name: flake8
 
-on: [push]
+on: [pull_request]
 
 jobs:
   build:

.github/workflows/python2.yml

 name: python2
 
-on: [push]
+on: [pull_request]
 
 jobs:
   build:

.github/workflows/runnable_submodules.yml

 name: runnable submodules
 
-on: [push]
+on: [pull_request]
 
 jobs:
   build:

.github/workflows/tools.yml

 name: tools
 
-on: [push]
+on: [pull_request]
 
 jobs:
   build:

.github/workflows/unittest.yml

 name: unit tests
 
-on: [push]
+on: [pull_request]
 
 jobs:
   build: