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/errors.py

@@ -57,7 +57,7 @@ def parse_errors(csv_file):
     Parses the input CSV file with columns (name, error codes, description)
     and yields `Error` instances as a result.
     """
-    with open(csv_file, newline='') as f:
+    with csv_file.open(newline='') as f:
         f = csv.reader(f)
         next(f, None)  # header
         for line, (name, codes, description) in enumerate(f, start=2):