`NameError` in `tinytex_download.py`

[zietzm]

Thanks for the nice project!

I'm trying to use pytinytex but am getting the following error:

NameError: name 'urllib' is not defined. Did you forget to import 'urllib'?

in reference to

PyTinyTeX/pytinytex/tinytex_download.py

Line 77 in aa36458

except urllib.error.HTTPError as e:

It looks like urllib is only imported in this block:

PyTinyTeX/pytinytex/tinytex_download.py

Lines 10 to 13 in aa36458

	try:
	from urllib.request import urlopen
	except ImportError:
	from urllib import urlopen

but should also be imported at the top level.

Happy to open a quick PR if desired. Thanks!

[JessicaTegner]

@zietzm feel free to open a pr, to clean it up