Fix coin change (#2571)

* Removed unused variable m.

* Doctests are modified to match functions.

* Added condition for negative values.

* Fixed white-space around operator.

* Fixed W293 blank line contains white-space error.

* Update dynamic_programming/coin_change.py

Co-authored-by: Tapajyoti Bose <44058757+ruppysuppy@users.noreply.github.com>

* Fixed error in code.

* Fixed whited spacing.

* Fixed PEP8 error.

* Added more test cases for coin change problem.

* Removed extra test for negetive value.

Co-authored-by: Tapajyoti Bose <44058757+ruppysuppy@users.noreply.github.com>