Remove limitation that seconds has two or fewer digits

- ISO 8601 doesn't seem to express that explicit limitations here are
  necessary.

Author: mgmarino

pandas/_libs/tslibs/timedeltas.pyx

@@ -662,11 +662,8 @@ cdef inline int64_t parse_iso_format_string(str ts) except? -1:
                     r = timedelta_from_spec(number, '0', dec_unit)
                     result += timedelta_as_neg(r, neg)
                 else:  # seconds
-                    if len(number) <= 2:
-                        r = timedelta_from_spec(number, '0', 'S')
-                        result += timedelta_as_neg(r, neg)
-                    else:
-                        raise ValueError(err_msg)
+                    r = timedelta_from_spec(number, '0', 'S')
+                    result += timedelta_as_neg(r, neg)
             else:
                 raise ValueError(err_msg)
 

pandas/tests/scalar/timedelta/test_constructors.py

@@ -234,6 +234,8 @@ def test_overflow_on_construction():
         ("P1D", Timedelta(days=1)),
         ("P1DT1H", Timedelta(days=1, hours=1)),
         ("P1W", Timedelta(days=7)),
+        ("PT300S", Timedelta(seconds=300)),
+        ("P1DT0H0M00000000000S", Timedelta(days=1)),
     ],
 )
 def test_iso_constructor(fmt, exp):
@@ -247,7 +249,6 @@ def test_iso_constructor(fmt, exp):
         "PDTHMS",
         "P0DT999H999M999S",
         "P1DT0H0M0.0000000000000S",
-        "P1DT0H0M00000000000S",
         "P1DT0H0M0.S",
     ],
 )