replace `setup.py` with `pyproject.toml`

[yunjunz]

Description of proposed changes

• add tests/requirements.txt file for test requirements

• requirements.txt: move pre-commit into the tests/requirements.txt file

• pyproject.toml: add all the corresponding contents from setup.py based on https://setuptools.pypa.io/en/latest/userguide/pyproject_config.html

• delete setup.py

Reminders

• [x] Pass Pre-commit check (green)
• [x] Pass Codacy code review (green)
• [x] Pass Circle CI test (green)
• [x] Make sure that your code follows our style. Use the other functions/files as a basis.
• [x] If modifying functionality, describe changes to function behavior and arguments in a comment below the function declaration.

[yunjunz]

I try to replace the old setup.py with pyproject.toml here, but testing both locally and in the Circle CI failed due to the same error as below. @avalentino @jhkennedy @scottstanie Could you take a look at the PR when you get a chance and help me find out what went wrong?

(test_mintpy) yunjunz:~>$ cd ~/tools/MintPy/
(test_mintpy) yunjunz:~/tools/MintPy>$ python -m pip install -e .
Obtaining file:///Users/yunjunz/tools/MintPy
  Installing build dependencies ... done
  Checking if build backend supports build_editable ... done
  Getting requirements to build editable ... done
  Preparing editable metadata (pyproject.toml) ... done
INFO: pip is looking at multiple versions of mintpy to determine which version is compatible with other requirements. This could take a while.
ERROR: Could not find a version that satisfies the requirement python>=3.6 (from mintpy) (from versions: none)
ERROR: No matching distribution found for python>=3.6

[scottstanie]

I think the python>=3.6 part isn't install-able via pip, so it had to be removed from requirements.txt. It's enforced by having it in pyproject.toml. (sorry I didn't mean to push directly to you branch! 😬 )

By the way, had you considered bumping up the min python version? Both 3.6 and 3.7 are now in the end-of-life stage

[yunjunz]

Thank you @scottstanie @avalentino for the quick fix and suggestions.

I just bumped the min version to 3.8 as you suggested. I will look at the new syntax of 3.7 and 3.8 on another day to see what can be simplified in the code to leverage the new version!