python: Convert assignment type annotations to Python 3.6 style.

Commit split by tabbott; this has changes to scripts/, tools/, and
puppet/.

scripts/lib/hash_reqs.py, scripts/lib/setup_venv.py,
scripts/lib/zulip_tools.py, and tools/lib/provision.py are excluded so
tools/provision still gives the right error message on Ubuntu 16.04
with Python 3.5.

Generated by com2ann, with whitespace fixes and various manual fixes
for runtime issues:

-shebang_rules: List[Rule] = [
+shebang_rules: List["Rule"] = [

-trailing_whitespace_rule: Rule = {
+trailing_whitespace_rule: "Rule" = {

-whitespace_rules: List[Rule] = [
+whitespace_rules: List["Rule"] = [

-comma_whitespace_rule: List[Rule] = [
+comma_whitespace_rule: List["Rule"] = [

-prose_style_rules: List[Rule] = [
+prose_style_rules: List["Rule"] = [

-html_rules: List[Rule] = whitespace_rules + prose_style_rules + [
+html_rules: List["Rule"] = whitespace_rules + prose_style_rules + [

-    target_port: int = None
+    target_port: int

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

tools/lib/pretty_print.py

@@ -17,23 +17,24 @@ def pretty_print_html(html: str, num_spaces: int = 4) -> str:
     # We will keep a stack of "start" tags so that we know
     # when HTML ranges end.  Note that some start tags won't
     # be blocks from an indentation standpoint.
-    stack = []  # type: List[Dict[str, Any]]
+    stack: List[Dict[str, Any]] = []
 
     # Seed our stack with a pseudo entry to make depth calculations
     # easier.
-    info = dict(
+    info: Dict[str, Any] = dict(
         block=False,
         depth=-1,
         line=-1,
         token_kind='html_start',
         tag='html',
         extra_indent=0,
-        ignore_lines=[])  # type: Dict[str, Any]
+        ignore_lines=[],
+    )
     stack.append(info)
 
     # Our main job is to figure out offsets that we use to nudge lines
     # over by.
-    offsets = {}  # type: Dict[int, int]
+    offsets: Dict[int, int] = {}
 
     # Loop through our start/end tokens, and calculate offsets.  As
     # we proceed, we will push/pop info dictionaries on/off a stack.