Fix for the bug #7343

Corrected the bug by adding solutions returned from
each individual equation to the `sols` set. Previous
error was due to replacing the last solution in `sols`
by the new solution rather than appending it to the `sols`.
Fix for the issue #7342

Change the return type of `diop_solve()` to a tuple in the univariable
case. Also added the checks to ensure that only  `Add`, `Mul` or a
`Symbol` is processed in the `classify_diop()` function. Raised an
error in the other cases.
Replace `==` by `is`
Fixed merge conflicts.

Conflicts:
	sympy/solvers/diophantine.py