Add DEFAULT_AUGUR_RECURSION_LIMIT=10000

We've had to use the `AUGUR_RECURSION_LIMIT` environment variable to
bump the recursion limit for SARS-CoV-2 builds¹, mpox builds², and
our `nextstrain-jobs` definition in AWS Batch. From a general search of
the envvar across GitHub³, 10,000 looks like the default that others use
as well (although this may be influenced by our discussions⁴).

¹ nextstrain/ncov#690
² nextstrain/mpox#150
³ https://cs.github.com/?scopeName=All+repos&scope=&q=AUGUR_RECURSION_LIMIT%3D
⁴ #328

augur/__init__.py

@@ -15,9 +15,8 @@
 from .io.print import print_err
 from .argparse_ import add_command_subparsers, add_default_command
 
-recursion_limit = os.environ.get("AUGUR_RECURSION_LIMIT")
-if recursion_limit:
-    sys.setrecursionlimit(int(recursion_limit))
+DEFAULT_AUGUR_RECURSION_LIMIT = 10000
+sys.setrecursionlimit(int(os.environ.get("AUGUR_RECURSION_LIMIT", DEFAULT_AUGUR_RECURSION_LIMIT)))
 
 command_strings = [
     "parse",