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/html_grep.py

@@ -7,7 +7,7 @@ def show_all_branches(fns):
     # type: (List[str]) -> None
     for fn in fns:
         print(fn)
-        with open(fn, 'r') as f:
+        with open(fn) as f:
             text = f.read()
         branches = html_branches(text, fn=fn)
         for branch in branches:
@@ -26,7 +26,7 @@ class Grepper:
         all_branches = []  # type: List[HtmlTreeBranch]
 
         for fn in fns:
-            with open(fn, 'r') as f:
+            with open(fn) as f:
                 text = f.read()
             branches = html_branches(text, fn=fn)
             all_branches += branches