Fix Sphinx intersphinx_mapping configuration

Fix Sphinx build error: 'Invalid intersphinx_mapping configuration'

**Issues fixed:**
1. Removed duplicate 'py3' and 'python' entries (kept 'python')
2. Removed obsolete 'rtd' reference (readthedocs.io docs)
3. Updated numpy: http://docs.scipy.org/doc/numpy → https://numpy.org/doc/stable/
4. Removed scipy and matplotlib (not cfxdb dependencies)

**Modern intersphinx_mapping:**
- python: Python 3 standard library docs
- txaio: txaio library docs
- autobahn: Autobahn|Python docs
- zlmdb: zLMDB docs
- numpy: NumPy docs (used for array types in cfxdb)

**Why these changes:**
- Modern Sphinx versions are stricter about URL validation
- HTTPS preferred over HTTP
- Old scipy.org URLs for numpy are deprecated
- Only include relevant project dependencies

This aligns with Sphinx best practices and cfxdb's actual dependencies.

Author: oberstet

docs/conf.py

@@ -164,13 +164,9 @@ def use_aio():
 html_static_path = ['_static']
 
 intersphinx_mapping = {
-    'py3': ('https://docs.python.org/3', None),
     'python': ('https://docs.python.org/3', None),
-    'rtd': ('https://docs.readthedocs.io/en/latest/', None),
     'txaio': ('https://txaio.readthedocs.io/en/latest/', None),
     'autobahn': ('https://autobahn.readthedocs.io/en/latest/', None),
     'zlmdb': ('https://zlmdb.readthedocs.io/en/latest/', None),
-    'numpy': ('http://docs.scipy.org/doc/numpy', None),
-    'scipy': ('http://docs.scipy.org/doc/scipy/reference', None),
-    'matplotlib': ('http://matplotlib.org/stable', None),
+    'numpy': ('https://numpy.org/doc/stable/', None),
 }