Reconfigure arrow/__init__.py to avoid mypy --strict problems (#938)

* Explicit reexport values

Mypy prevents the implicit reexport, when --no-implicit-reexport is given.

* Add missing ParserError and add comment

* Fix typo in comment

arrow/__init__.py

@@ -15,3 +15,25 @@
     FORMAT_W3C,
 )
 from .parser import ParserError
+
+# https://mypy.readthedocs.io/en/stable/command_line.html#cmdoption-mypy-no-implicit-reexport
+# Mypy with --strict or --no-implicit-reexport requires an explicit reexport.
+__all__ = [
+    "__version__",
+    "get",
+    "now",
+    "utcnow",
+    "Arrow",
+    "ArrowFactory",
+    "FORMAT_ATOM",
+    "FORMAT_COOKIE",
+    "FORMAT_RFC822",
+    "FORMAT_RFC850",
+    "FORMAT_RFC1036",
+    "FORMAT_RFC1123",
+    "FORMAT_RFC2822",
+    "FORMAT_RFC3339",
+    "FORMAT_RSS",
+    "FORMAT_W3C",
+    "ParserError",
+]