Make use of pathlib nearly everywhere (breaks docs gen)

Python 3.6 introduced support for the os.PathLike interface,
which means Python 3.5 did not have it yet and attempting to
use it in os functions would fail. Instead we can use pathlib
for everything, but not all work is done yet.

telethon_generator/parsers/tlobject/parser.py

@@ -86,7 +86,7 @@ def parse_tl(file_path, layer, methods=None, ignored_ids=CORE_TYPES):
     obj_all = []
     obj_by_name = {}
     obj_by_type = collections.defaultdict(list)
-    with open(file_path, 'r', encoding='utf-8') as file:
+    with file_path.open() as file:
         is_function = False
         for line in file:
             comment_index = line.find('//')