Fix issues reported by codacy

Dockerfile

@@ -1,6 +1,6 @@
 FROM python:3.6.10-alpine3.10
 
-RUN pip3 install pygithub
+RUN pip3 install pygithub==1.47
 
 COPY verify_pr_lables.py /verify_pr_lables.py
 

README.md

@@ -12,26 +12,26 @@ Instead, consider the same example, the action is triggered by `pull_request`'s
 
 ## Inputs
 
-### `github-token`:
+### `github-token`
 
 **Required** The GitHub token.
 
-### `valid-labels`:
+### `valid-labels`
 
 **Required** A list of valid labels. It must be a quoted string, with label separated by colons. For example: `'bug, enhancement'`
 
 ## Example usage
 
 In your workflow YAML file add this step:
-```
+```yaml
 uses: actions/[email protected]
 with:
     github-token: '${{ secrets.GITHUB_TOKEN }}'
     valid-labels: 'bug, enhancement'
 ```
 
 and trigger it with:
-```
+```yaml
 on:
   pull_request:
    types: [opened, labeled, unlabeled, synchronize]

verify_pr_lables.py

@@ -7,6 +7,7 @@
 
 def get_env_var(env_var_name, echo_value=False):
     """Try to get the value from a environmental variable.
+
     If the values is 'None', then a ValueError exception will
     be thrown.