test: from functools import reduce in test/testpy/__init__.py

$ __make lint-py__  # When run on Python 3
```
PYTHONPATH=tools/pip python -m flake8 . \
		--count --show-source --statistics --select=E901,E999,F821,F822,F823 \
		--exclude=.git,deps,lib,src,tools/*_macros.py,tools/gyp,tools/inspector_protocol,tools/jinja2,tools/markupsafe,tools/pip
./test/testpy/__init__.py:119:37: F821 undefined name 'reduce'
        file_path = join(self.root, reduce(join, test[1:], ""))
                                    ^
./test/testpy/__init__.py:161:37: F821 undefined name 'reduce'
        file_path = join(self.root, reduce(join, test[1:], "") + ".js")
                                    ^
2     F821 undefined name 'reduce'
2
make: *** [lint-py] Error 1
```

PR-URL: #24954
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Anna Henningsen <anna@addaleax.net>

test/testpy/__init__.py

@@ -31,6 +31,11 @@
 import re
 import ast
 
+try:
+  reduce
+except NameError:
+  from functools import reduce
+
 
 FLAGS_PATTERN = re.compile(r"//\s+Flags:(.*)")
 FILES_PATTERN = re.compile(r"//\s+Files:(.*)")