Make implicit imports explicit

Closes #825

Implicit imports (imports that rely on the fact
that importing a module imports another module
such as a submodule) makes the code vulnerable
to breakage due to refactors in the dependent
code. Such as the changes in Sphinx 8.2.0 that
caused #825

This replaces all implicit imports that I
could find by explicit imports.

Author: jenshnielsen

src/nbsphinx/__init__.py

@@ -8,6 +8,7 @@
 import collections.abc
 import copy
 import html
+import html.parser
 from itertools import chain
 import json
 import os
@@ -18,23 +19,42 @@
 import uuid
 
 import docutils
+import docutils.nodes
+import docutils.transforms
+import docutils.frontend
+import docutils.utils
 from docutils.parsers import rst
+from docutils.parsers.rst import directives as rst_directives
 import jinja2
 import nbconvert
+import nbconvert.utils
+import nbconvert.utils.pandoc
+import nbconvert.utils.version
 import nbformat
 import sphinx
+import sphinx.addnodes
 import sphinx.directives
+import sphinx.directives.code
 import sphinx.directives.other
 import sphinx.environment
+import sphinx.environment.adapters.toctree
 import sphinx.errors
 import sphinx.transforms.post_transforms.images
+import sphinx.util
+import sphinx.util.console
+import sphinx.util.fileutil
+import sphinx.util.images
+import sphinx.util.logging
+import sphinx.util.nodes
+import sphinx.util.osutil
 from sphinx.util.matching import patmatch
 try:
     from sphinx.util.display import status_iterator
 except ImportError:
     # This will be removed in Sphinx 8:
     from sphinx.util import status_iterator
 import traitlets
+import traitlets.config
 
 
 if sys.version_info >= (3, 8) and sys.platform == 'win32':
@@ -782,10 +802,10 @@ class NbInput(rst.Directive):
     optional_arguments = 1  # lexer name
     final_argument_whitespace = False
     option_spec = {
-        'execution-count': rst.directives.positive_int,
-        'empty-lines-before': rst.directives.nonnegative_int,
-        'empty-lines-after': rst.directives.nonnegative_int,
-        'no-output': rst.directives.flag,
+        'execution-count': rst_directives.positive_int,
+        'empty-lines-before': rst_directives.nonnegative_int,
+        'empty-lines-after': rst_directives.nonnegative_int,
+        'no-output': rst_directives.flag,
     }
     has_content = True
 
@@ -800,10 +820,10 @@ class NbOutput(rst.Directive):
     required_arguments = 0
     final_argument_whitespace = False
     option_spec = {
-        'execution-count': rst.directives.positive_int,
-        'more-to-come': rst.directives.flag,
-        'fancy': rst.directives.flag,
-        'class': rst.directives.unchanged,
+        'execution-count': rst_directives.positive_int,
+        'more-to-come': rst_directives.flag,
+        'fancy': rst_directives.flag,
+        'class': rst_directives.unchanged,
     }
     has_content = True
 
@@ -867,10 +887,10 @@ class NbLinkGallery(NbGallery):
 
     # Not all options of TocTree are allowed:
     option_spec = {
-        'name': rst.directives.unchanged,
-        'caption': rst.directives.unchanged_required,
-        'glob': rst.directives.flag,
-        'reversed': rst.directives.flag,
+        'name': rst_directives.unchanged,
+        'caption': rst_directives.unchanged_required,
+        'glob': rst_directives.flag,
+        'reversed': rst_directives.flag,
     }
 
 
@@ -2089,7 +2109,7 @@ def setup(app):
     # Make docutils' "code" directive (generated by markdown2rst/pandoc)
     # behave like Sphinx's "code-block",
     # see https://github.com/sphinx-doc/sphinx/issues/2155:
-    rst.directives.register_directive('code', sphinx.directives.code.CodeBlock)
+    rst_directives.register_directive('code', sphinx.directives.code.CodeBlock)
 
     # Monkey-patch Sphinx TocTree adapter
     if hasattr(sphinx.environment.adapters.toctree, '_resolve_toctree'):