Fix some open calls were not being made through pathlib

This was causing the documentation to fail to build under Python 3.5.

telethon_generator/docswriter.py

@@ -288,7 +288,7 @@ class DocsWriter:
     def __enter__(self):
         # Sanity check
         self.filename.parent.mkdir(parents=True, exist_ok=True)
-        self.handle = open(self.filename, 'w', encoding='utf-8')
+        self.handle = self.filename.open('w', encoding='utf-8')
         return self
 
     def __exit__(self, exc_type, exc_val, exc_tb):

telethon_generator/generators/docs.py

@@ -196,7 +196,7 @@ def _get_description(arg):
 
 def _copy_replace(src, dst, replacements):
     """Copies the src file into dst applying the replacements dict"""
-    with open(src, 'r') as infile, open(dst, 'w') as outfile:
+    with src.open() as infile, dst.open('w') as outfile:
         outfile.write(re.sub(
             '|'.join(re.escape(k) for k in replacements),
             lambda m: str(replacements[m.group(0)]),

telethon_generator/parsers/tlobject/parser.py

@@ -137,7 +137,7 @@ def parse_tl(file_path, layer, methods=None, ignored_ids=CORE_TYPES):
 def find_layer(file_path):
     """Finds the layer used on the specified scheme.tl file."""
     layer_regex = re.compile(r'^//\s*LAYER\s*(\d+)$')
-    with open(file_path, 'r', encoding='utf-8') as file:
+    with file_path.open('r') as file:
         for line in file:
             match = layer_regex.match(line)
             if match: