Sympy symbol names should not use '`' as context marks

[mmatera]

Motivated by the good results obtained in using sympy lambdify for compiling expressions, I have been looking at the code that converts Mathics expression to sympy. One problem I found is that when I convert an expression to sympy, symbol and function names include the '' character, which in Mathics is used to split contexts. In the sympy side, it looks OK until we try to produce source code (for example, when we try to use ufuncify). Ideally, context in sympy variables should be separated by a harmless character like _`.