Replace pkg_resources with importlib.metadata (#1674)

Use `importlib.metadata` from the standard library
(added in Python 3.8) to retrieve the package version
at runtime. A bit faster than using the `pkg_resources`
which adds some overhead and sets a runtime
dependency on `setuptools`.

* rename version variables in __init__.py

Co-authored-by: TIAN Dongdong <seisman.info@gmail.com>

Author: michaelgrund

pygmt/__init__.py

@@ -19,8 +19,7 @@
 """
 
 import atexit as _atexit
-
-from pkg_resources import get_distribution
+from importlib.metadata import version
 
 # Import modules to make the high-level GMT Python API
 from pygmt import datasets
@@ -63,7 +62,7 @@
 )
 
 # Get semantic version through setuptools-scm
-__version__ = f'v{get_distribution("pygmt").version}'  # e.g. v0.1.2.dev3+g0ab3cd78
+__version__ = f'v{version("pygmt")}'  # e.g. v0.1.2.dev3+g0ab3cd78
 __commit__ = __version__.split("+g")[-1] if "+g" in __version__ else ""  # 0ab3cd78
 
 # Start our global modern mode session
@@ -135,10 +134,9 @@ def _get_ghostscript_version():
 
         for gs_cmd in cmds:
             try:
-                version = subprocess.check_output(
+                return subprocess.check_output(
                     [gs_cmd, "--version"], universal_newlines=True
                 ).strip()
-                return version
             except FileNotFoundError:
                 continue
         return None
@@ -148,10 +146,9 @@ def _get_gmt_version():
         Get GMT version.
         """
         try:
-            version = subprocess.check_output(
+            return subprocess.check_output(
                 ["gmt", "--version"], universal_newlines=True
             ).strip()
-            return version
         except FileNotFoundError:
             return None