Representation of pd.Timestamp is not ISO8601 compliant.
ISO 8601 specifies that the date and time portion of a timestamp is separated by a "T", but python's datetime.isoformat allows any character to separate them. To comply with ISO8601, [line 650](https://github.com/pandas-dev/pandas/blob/6b2d0260c818e62052eaf535767f3a8c4b446c69/pandas/_libs/tslibs/timestamps.pyx#L650) should be changed to include a "T" in between the date and the time. Since this also complies with datetime.isoformat, there is no reason not to change this.