Fix sphinx errors and warnings

Author: aptiko

docs/API.rst

@@ -43,10 +43,6 @@ The base meta class is a PluginMount:
 .. autoclass:: pydifact.api.PluginMount
 
 
-Available entry points for plugins are:
-
-.. autoclass:: pydifact.segments.SegmentProvider
-    :members:
-
-    .. automethod:: SegmentProvider.__str__
-
+:class:`~pydifact.segments.SegmentProvider` uses
+:class:`~pydifact.api.PluginMount` and can thus be
+extended with plugins.

docs/conf.py

@@ -47,7 +47,6 @@
 html_theme = "sphinx_rtd_theme"
 
 html_theme_options = {
-    "canonical_url": "https://pydifact.readthedocs.io/",
     # "logo_only": False,
     "display_version": True,
     "prev_next_buttons_location": "both",

pydifact/segmentcollection.py

@@ -352,12 +352,12 @@ def validate(self):
 
 class Message(AbstractSegmentsContainer):
     """
-    A message (started by UNH segment, ended by UNT segment)
+    A message (started by UNH_ segment, ended by UNT_ segment)
 
     Optional features of UNH are not yet supported.
 
-    https://www.stylusstudio.com/edifact/40100/UNH_.htm
-    https://www.stylusstudio.com/edifact/40100/UNT_.htm
+    .. _UNH: https://www.stylusstudio.com/edifact/40100/UNH_.htm
+    .. _UNT: https://www.stylusstudio.com/edifact/40100/UNT_.htm
     """
 
     HEADER_TAG = "UNH"
@@ -407,7 +407,7 @@ def validate(self):
 
 class Interchange(FileSourcableMixin, UNAHandlingMixin, AbstractSegmentsContainer):
     """
-    An interchange (started by UNB segment, ended by UNZ segment)
+    An interchange (started by UNB_ segment, ended by UNZ_ segment)
 
     Optional features of UNB are not yet supported.
 
@@ -417,8 +417,8 @@ class Interchange(FileSourcableMixin, UNAHandlingMixin, AbstractSegmentsContaine
     optional: interchange segments can be accessed without going through
     messages.
 
-    https://www.stylusstudio.com/edifact/40100/UNB_.htm
-    https://www.stylusstudio.com/edifact/40100/UNZ_.htm
+    .. _UNB: https://www.stylusstudio.com/edifact/40100/UNB_.htm
+    .. _UNZ: https://www.stylusstudio.com/edifact/40100/UNZ_.htm
     """
 
     HEADER_TAG = "UNB"