Add type annotations to the majority of variables and functions in Service V2

Before developing the Service V4 I wanted to fully understand how V2 works, however without
data types it was nearly impossible to grasp the meaning of the code. Hence, this commit.

Author: mamiksik

CHANGELOG.md

@@ -20,6 +20,7 @@ and this project adheres to [Semantic Versioning](http://semver.org/).
 - Permissive parsing for TypeDefinition
 - Changes all manually raised exception to be child of PyODataException - Martin Miksik
 - More comprehensive tests for ODATA V4 - Martin Miksik
+- Majority of variables and functions in Service V2 are now type annotated - Martin Miksik
 
 ### Changed
 - Implementation and naming schema of `from_etree` - Martin Miksik

pyodata/exceptions.py

@@ -1,4 +1,5 @@
 """PyOData exceptions hierarchy"""
+import requests
 
 
 class PyODataException(Exception):
@@ -25,13 +26,13 @@ class HttpError(PyODataException):
 
     VendorType = None
 
-    def __new__(cls, message, response):
+    def __new__(cls, message: str, response: requests.Response) -> 'HttpError':
         if HttpError.VendorType is not None:
             return super(HttpError, cls).__new__(HttpError.VendorType, message, response)
 
-        return super(HttpError, cls).__new__(cls, message, response)
+        return super(HttpError, cls).__new__(cls, message, response)  # type: ignore
 
-    def __init__(self, message, response):
+    def __init__(self, message: str, response: requests.Response) -> None:
         super(HttpError, self).__init__(message)
 
         self.response = response