Fix pathlib support for plot and plot3d

[seisman]

Description of proposed changes

Minimal example to reproduce the issue:

import pygmt
from pathlib import Path

fig = pygmt.Figure()
fig.plot(data=Path("abc.txt"))
fig.show()

Error messages:

Traceback (most recent call last):
  File "/Users/user/pygmt/issues/plot-pathlib/test.py", line 5, in <module>
    fig.plot(data=Path("abc.txt"))
  File "/Users/user/pygmt/pygmt/helpers/decorators.py", line 872, in new_module
    return module_func(*args, **kwargs)
  File "/Users/user/pygmt/pygmt/helpers/decorators.py", line 585, in new_module
    return module_func(*args, **kwargs)
  File "/Users/user/pygmt/pygmt/helpers/decorators.py", line 725, in new_module
    return module_func(*args, **kwargs)
  File "/Users/user/pygmt/pygmt/src/plot.py", line 229, in plot
    elif "S" not in kwargs and kind == "file" and data.endswith(".gmt"):
AttributeError: 'PosixPath' object has no attribute 'endswith'

This PR fixes the bug.

Reminders

• Run make format and make check to make sure the code follows the style guide.
• 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.

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