Documentation build fails on readthedocs

[MassimoCimmino]

Here is the log:

Running Sphinx v1.8.5

Traceback (most recent call last):
  File "/home/docs/checkouts/readthedocs.org/user_builds/pygfunction/envs/latest/lib/python2.7/site-packages/sphinx/cmd/build.py", line 303, in build_main
    args.tags, args.verbosity, args.jobs, args.keep_going)
  File "/home/docs/checkouts/readthedocs.org/user_builds/pygfunction/envs/latest/lib/python2.7/site-packages/sphinx/application.py", line 201, in __init__
    self.config = Config.read(self.confdir, confoverrides or {}, self.tags)
  File "/home/docs/checkouts/readthedocs.org/user_builds/pygfunction/envs/latest/lib/python2.7/site-packages/sphinx/config.py", line 203, in read
    namespace = eval_config_file(filename, tags)
  File "/home/docs/checkouts/readthedocs.org/user_builds/pygfunction/envs/latest/lib/python2.7/site-packages/sphinx/config.py", line 373, in eval_config_file
    raise ConfigError(msg % err)
ConfigError: There is a syntax error in your configuration file: invalid syntax (boreholes.py, line 140)

Configuration error:
There is a syntax error in your configuration file: invalid syntax (boreholes.py, line 140)
Command time: 0s Return: 2

The reference is to:

pygfunction/pygfunction/boreholes.py

Line 140 in d48f310

output = f"{ '*gt.boreholes.find_duplicates()*' :-^50}"

It may also fail on the next few lines:

pygfunction/pygfunction/boreholes.py

Line 186 in d48f310

print(f"{'*gt.boreholes.remove_duplicates()*' :-^50}".\

[MassimoCimmino]

Another option to format the string is : ' gt.boreholes.find_duplicates() '.center(50, '-')

I don't know why Sphinx does not like the initial implementation (it works fine on my machine).

[MassimoCimmino]

The build later failed on:

pygfunction/pygfunction/media.py

Lines 67 to 68 in d48f310

	def __init__(self, mixer: str, percent: float,
	T: float = 20., P: float = 101325.,):

I found that readthedocs was building the docs using Python 2, which does not support function annotations. A configuration file is introduced in d4fa4a3.