Set setuptools_scm fallback_version to follow PEP440

[seisman]

Description of proposed changes

Address comment #1784 (comment) and #1848 (comment).

To test it, try:

pip install https://github.com/GenericMappingTools/pygmt/archive/refs/heads/fallback_version.zip

[weiji14]

Cool, seems to work ok now, pip install https://github.com/GenericMappingTools/pygmt/archive/refs/heads/fallback_version.zip:

Collecting https://github.com/GenericMappingTools/pygmt/archive/refs/heads/fallback_version.zip
  Downloading https://github.com/GenericMappingTools/pygmt/archive/refs/heads/fallback_version.zip
     | 3.9 MB 4.2 MB/s 0:00:00
  Installing build dependencies ... done
  Getting requirements to build wheel ... done
  Installing backend dependencies ... done
  Preparing metadata (pyproject.toml) ... done
Requirement already satisfied: numpy>=1.19 in /home/username/mambaforge/envs/pygmt/lib/python3.9/site-packages (from pygmt==999.999.999+unknown) (1.22.4)
Collecting netCDF4
  Using cached netCDF4-1.5.8-cp39-cp39-manylinux_2_17_x86_64.manylinux2014_x86_64.whl (4.7 MB)
Requirement already satisfied: xarray in /home/username/mambaforge/envs/pygmt/lib/python3.9/site-packages (from pygmt==999.999.999+unknown) (2022.3.0)
Requirement already satisfied: packaging in /home/username/mambaforge/envs/pygmt/lib/python3.9/site-packages (from pygmt==999.999.999+unknown) (21.3)
Requirement already satisfied: pandas in /home/username/mambaforge/envs/pygmt/lib/python3.9/site-packages (from pygmt==999.999.999+unknown) (1.4.2)
Collecting cftime
  Using cached cftime-1.6.0-cp39-cp39-manylinux_2_17_x86_64.manylinux2014_x86_64.whl (239 kB)
Requirement already satisfied: pyparsing!=3.0.5,>=2.0.2 in /home/username/mambaforge/envs/pygmt/lib/python3.9/site-packages (from packaging->pygmt==999.999.999+unknown) (3.0.9)
Requirement already satisfied: pytz>=2020.1 in /home/username/mambaforge/envs/pygmt/lib/python3.9/site-packages (from pandas->pygmt==999.999.999+unknown) (2022.1)
Requirement already satisfied: python-dateutil>=2.8.1 in /home/username/mambaforge/envs/pygmt/lib/python3.9/site-packages (from pandas->pygmt==999.999.999+unknown) (2.8.2)
Requirement already satisfied: six>=1.5 in /home/username/mambaforge/envs/pygmt/lib/python3.9/site-packages (from python-dateutil>=2.8.1->pandas->pygmt==999.999.999+unknown) (1.16.0)
Building wheels for collected packages: pygmt
  Building wheel for pygmt (pyproject.toml) ... done
  Created wheel for pygmt: filename=pygmt-999.999.999+unknown-py3-none-any.whl size=346893 sha256=0bcf92ecfbd89c9cb22e42497a222472edd5a521cc56eaca75115482b20c67b7
  Stored in directory: /tmp/pip-ephem-wheel-cache-0kntmfs0/wheels/bc/33/c2/8f3051bbe2f3d0737fe8d1dc5baa0e42e04ba7cd1c44947942
Successfully built pygmt
Installing collected packages: cftime, netCDF4, pygmt
Successfully installed cftime-1.6.0 netCDF4-1.5.8 pygmt-999.999.999+unknown