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.

setup.py

@@ -16,6 +16,7 @@ import re
 import shutil
 from os import chdir
 from pathlib import Path
+from subprocess import run
 from sys import argv
 
 from setuptools import find_packages, setup
@@ -30,11 +31,11 @@ class TempWorkDir:
 
     def __enter__(self):
         self.original = Path('.')
-        chdir(Path(__file__).parent)
+        chdir(str(Path(__file__).parent))
         return self
 
     def __exit__(self, *args):
-        chdir(self.original)
+        chdir(str(self.original))
 
 
 GENERATOR_DIR = Path('telethon_generator')
@@ -96,7 +97,7 @@ def generate(which):
             if ERRORS_OUT.is_file():
                 ERRORS_OUT.unlink()
         else:
-            with open(ERRORS_OUT, 'w', encoding='utf-8') as file:
+            with ERRORS_OUT.open('w') as file:
                 generate_errors(errors, file)
 
     if 'docs' in which:
@@ -104,7 +105,7 @@ def generate(which):
         print(action, 'documentation...')
         if clean:
             if DOCS_OUT.is_dir():
-                shutil.rmtree(DOCS_OUT)
+                shutil.rmtree(str(DOCS_OUT))
         else:
             generate_docs(tlobjects, methods, layer, DOCS_IN_RES, DOCS_OUT)
 
@@ -154,18 +155,13 @@ def main():
             print('Packaging for PyPi aborted, importing the module failed.')
             return
 
-        # Need python3.5 or higher, but Telethon is supposed to support 3.x
-        # Place it here since noone should be running ./setup.py pypi anyway
-        from subprocess import run
-        from shutil import rmtree
-
         for x in ('build', 'dist', 'Telethon.egg-info'):
-            rmtree(x, ignore_errors=True)
+            shutil.rmtree(x, ignore_errors=True)
         run('python3 setup.py sdist', shell=True)
         run('python3 setup.py bdist_wheel', shell=True)
         run('twine upload dist/*', shell=True)
         for x in ('build', 'dist', 'Telethon.egg-info'):
-            rmtree(x, ignore_errors=True)
+            shutil.rmtree(x, ignore_errors=True)
 
     else:
         # e.g. install from GitHub