avoid underscore numeral syntax

Numerals like 1_000_000 are supported in Python 3.6, but we're currently
testing as far back as 3.5.

tests/test_blake3.py

@@ -125,7 +125,7 @@ def test_xof():
 
 
 def test_multithreading():
-    b = make_input(1_000_000)
+    b = make_input(10 ** 6)
     expected = blake3(b).digest()
     assert expected == blake3(b, multithreading=True).digest()
     incremental = blake3()