Index.insert(-1, value) should add value in the last position
#### Problem description and Expected Output

``Index.insert(-1, value)`` should add value in the last position instead of one before the last position, shouldn't it?

#### Output of ``pd.show_versions()``

<details>

INSTALLED VERSIONS
------------------
commit           : None
python           : 3.7.6.final.0
python-bits      : 64
OS               : Windows
OS-release       : 10
machine          : AMD64
processor        : Intel64 Family 6 Model 142 Stepping 10, GenuineIntel
byteorder        : little
LC_ALL           : None
LANG             : en
LOCALE           : None.None

pandas           : 1.0.1
numpy            : 1.18.1

</details>
