build: always run waf with python3

Upstream waf still ships with the default interpreter being "python",
though the script works with both Python 2 and Python 3 (they're not
changing the default choice during 2.0.x releases for compatibility
reasons apparently). Add code to bootstrap.py to change the
interpreter from "python" to "python3" when downloading the "waf"
file.

Running any mpv code under Python 2 should be considered unsupported
in the future (and any code added need not work under Python 2).

bootstrap.py

@@ -41,6 +41,12 @@ if not waf:
     sys.exit(1)
 
 if SHA256HASH == hashlib.sha256(waf).hexdigest():
+    # Upstream waf is not changing the default interpreter during
+    # 2.0.x line due to compatibility reasons apparently. So manually
+    # convert it to use python3 (the script works with both).
+    expected = b"#!/usr/bin/env python\n"
+    assert waf.startswith(expected)
+    waf = b"#!/usr/bin/env python3\n" + waf[len(expected):]
     with open("waf", "wb") as wf:
         wf.write(waf)