Let pygmt.info load datetime columns into a str dtype array (#960)

Fixes problem with pygmt.info not being able to handle datetime64 inputs.
I.e. `ValueError: could not convert string to float: '2021-01-01T12:34:56'`
However, users will still need to use`pygmt.info(..., coltypes="0T")` until
upstream issue at GenericMappingTools/gmt#4241
is resolved. Also added two extra unit tests using numpy datetime64 types.

* Use common alias coltypes (f) in tests and make a note on the workaround

Author: weiji14

pygmt/src/info.py

@@ -74,6 +74,10 @@ def info(table, **kwargs):
             # instead of a raw string that is less useful.
             if result.startswith(("-R", "-T")):  # e.g. -R0/1/2/3 or -T0/9/1
                 result = result[2:].replace("/", " ")
-            result = np.loadtxt(result.splitlines())
+            try:
+                result = np.loadtxt(result.splitlines())
+            except ValueError:
+                # Load non-numerical outputs in str type, e.g. for datetime
+                result = np.loadtxt(result.splitlines(), dtype="str")
 
         return result

pygmt/tests/test_info.py

@@ -41,6 +41,20 @@ def test_info_dataframe():
     assert output == expected_output
 
 
+def test_info_numpy_array_time_column():
+    """
+    Make sure info works on a numpy.ndarray input with a datetime type.
+    """
+    table = pd.date_range(start="2020-01-01", periods=5).to_numpy()
+    # Please remove coltypes="0T" workaround after
+    # https://github.com/GenericMappingTools/gmt/issues/4241 is resolved
+    output = info(table=table, coltypes="0T")
+    expected_output = (
+        "<vector memory>: N = 5 <2020-01-01T00:00:00/2020-01-05T00:00:00>\n"
+    )
+    assert output == expected_output
+
+
 @pytest.mark.xfail(
     reason="UNIX timestamps returned instead of ISO datetime, should work on GMT 6.2.0 "
     "after https://github.com/GenericMappingTools/gmt/issues/4241 is resolved",
@@ -115,6 +129,19 @@ def test_info_per_column():
     )
 
 
+def test_info_per_column_with_time_inputs():
+    """
+    Make sure the per_column option works with time inputs.
+    """
+    table = pd.date_range(start="2020-01-01", periods=5).to_numpy()
+    # Please remove coltypes="0T" workaround after
+    # https://github.com/GenericMappingTools/gmt/issues/4241 is resolved
+    output = info(table=table, per_column=True, coltypes="0T")
+    npt.assert_equal(
+        actual=output, desired=["2020-01-01T00:00:00", "2020-01-05T00:00:00"]
+    )
+
+
 def test_info_spacing():
     """
     Make sure the spacing option works.