Fix leading underscore in converted dummy name by slicing

[ayushk7102]

Possible fix for #372 (leading underscore appearing in converted dummy type)
Modified line 1265 from:
return sympy.Dummy(str(self))
to
return sympy.Dummy(str(self)[1:]

[ayushk7102]

I'm getting the error in the AppVeyor build:
CMake Error at C:/Program Files (x86)/CMake/share/cmake-3.16/Modules/FindPackageHandleStandardArgs.cmake:146 (message): Could NOT find GMP (missing: GMP_LIBRARIES GMP_INCLUDE_DIRS)
Is there anything I can do to fix this?

[isuruf]

Looks good to me. Can you add a test?

You can ignore the appveyor failure