python: Modernize legacy Python 2 syntax with pyupgrade.

Generated by `pyupgrade --py3-plus --keep-percent-format` on all our
Python code except `zthumbor` and `zulip-ec2-configure-interfaces`,
followed by manual indentation fixes.

Signed-off-by: Anders Kaseorg <anders@zulipchat.com>

tools/lib/template_parser.py

@@ -214,7 +214,7 @@ def validate(fn=None, text=None, check_indent=True):
         fn = '<in memory file>'
 
     if text is None:
-        with open(fn, 'r') as f:
+        with open(fn) as f:
             text = f.read()
 
     tokens = tokenize(text)