docs: Fix typing.Union warning

Fix the warning by pinning the `intersphinx_mapping` for Python to 3.13. I did 
not dig into why the Python 3.14 changes for typing causes the warning.¹

I had tried to pin to 3.9 which is our minimal supported Python version, but 
that led to other warnings:

```
/Users/joverlee/Repos/nextstrain/augur/augur/io/shell_command_runner.py:docstring of augur.io.shell_command_runner.ShellCommandRunner.signal_from_error:1: WARNING: py:class reference target not found: signal.Signals [ref.class]
/Users/joverlee/Repos/nextstrain/augur/augur/types.py:docstring of augur.types.ArgparseEnum:4: WARNING: py:class reference target not found: enum.StrEnum [ref.class]
```

¹ <https://docs.python.org/3.14/whatsnew/3.14.html#typing>

Author: joverlee521

docs/conf.py

@@ -145,7 +145,7 @@ def prose_list(items):
     'Bio': ('https://biopython.org/docs/latest/', None),
     'docs.nextstrain.org': ('https://docs.nextstrain.org/en/latest/', None),
     'cli': ('https://docs.nextstrain.org/projects/cli/en/stable', None),
-    'python': ('https://docs.python.org/3', None),
+    'python': ('https://docs.python.org/3.13', None),
     'numpy': ('https://numpy.org/doc/stable', None),
     'pandas': ('https://pandas.pydata.org/docs', None),
     'treetime': ('https://treetime.readthedocs.io/en/stable/', None),