Improve tagging docs: do not show useless imports

docs/maintaining/tagging.md

@@ -52,7 +52,7 @@ All the taggers are inherited from `TaggerInterface`:
 
 ```{literalinclude} ../../tagging/taggers/tagger_interface.py
 :language: py
-:lines: 3-
+:start-at: class TaggerInterface
 ```
 
 So, the `tag_value(container)` method gets a docker container as an input and returns a tag.
@@ -61,7 +61,7 @@ So, the `tag_value(container)` method gets a docker container as an input and re
 
 ```{literalinclude} ../../tagging/taggers/sha.py
 :language: py
-:lines: 3-
+:start-at: class SHATagger
 ```
 
 - `taggers/` subdirectory contains all the taggers.
@@ -76,7 +76,7 @@ All the other manifest classes are inherited from `ManifestInterface`:
 
 ```{literalinclude} ../../tagging/manifests/manifest_interface.py
 :language: py
-:lines: 3-
+:start-at: class ManifestInterface
 ```
 
 - The `markdown_piece(container)` method returns a piece of markdown file to be used as a part of the build manifest.
@@ -85,7 +85,7 @@ All the other manifest classes are inherited from `ManifestInterface`:
 
 ```{literalinclude} ../../tagging/manifests/apt_packages.py
 :language: py
-:lines: 3-
+:start-at: class AptPackagesManifest
 ```
 
 - `quoted_output(container, cmd)` simply runs the command inside a container using `DockerRunner.run_simple_command` and wraps it to triple quotes to create a valid markdown piece.