Move variable __gmt_version__ to pygmt.clib to avoid cyclic-import errors

[seisman]

Description of proposed changes

Defining __gmt_version__ in pygmt/__init__.py causes cyclic-import errors.
This PR fixes the issue by moving __gmt_version__ from pygmt/__init__.py
to pygmt/clib/__init__.py.

Closes #2712.

Reminders

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• Add tests for new features or tests that would have caught the bug that you're fixing.
• Add new public functions/methods/classes to doc/api/index.rst.
• Write detailed docstrings for all functions/methods.
• If wrapping a new module, open a 'Wrap new GMT module' issue and submit reasonably-sized PRs.
• If adding new functionality, add an example to docstrings or tutorials.
• Use underscores (not hyphens) in names of Python files and directories.

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[weiji14]

Looking at how rasterio handles __gdal_version__ at https://github.com/rasterio/rasterio/blob/6bf045cc5e02c9f28e78d48b97b308490207d27d/rasterio/__init__.py#L84-L86, they do:

from rasterio._version import gdal_version

__gdal_version__ = gdal_version()

They have a _version.pyx and a _version.pxd file that seems to be reading from the gdal.h header file. Do you think we could implement something similar for PyGMT/GMT by looking at include/gmt_version.h?

[seisman]

The way we get the GMT version string doesn't matter. What matters is that the __gmt_version__ variable is defined in pygmt/__init__.py and is internally used in other files.

Take pygmt/src/timestamp.py as an example. It imports the __gmt_version__ variable from pygmt/__init__.py. However, pygmt/__init__.py imports Figure from pygmt/figure.py and pygmt/figure.py imports timestamp from pygmt/src/timestamp.py. Thus, there are always cyclic import issues if we define __gmt_version__ in pygmt/__init__.py.

The only solution I can see is defining __gmt_version__ in another file, like pygmt/clib/__init__.py in this PR, or a new file like pygmt/_version.py.

[weiji14]

Ok, let's keep things in pygmt/clib/__init__.py for now since it's all internal variables, to get the Style Checks passing. We can look into moving things later to _version.py if needed.