Added support for Poly(f, x, y).coeff(x*y)
update coeff-related docstrings; special case Poly in coeff
change Poly.coeff to Poly.monomial_coeff
change monomial_coeff to coeff_monomial
edit docstrings
Poly.coeff is NotImplemented
Fixed Issue 3576
Update sympy/core/symbol.py

Indentations are corrected.
Update sympy/core/tests/test_symbol.py

Test Cases for different types of Parentheses examples have been added.
symbols allows escaping of ,; and space

Also, only one layer of surrounding parentheses are removed from
ranges, so if they are doubled, one pair will remain:

symbols('x((1:3))') -> (x(1), x(2))
Add Akshit Agarwal to AUTHORS/aboutus. Welcome to SymPy!
use string.ascii_letters for Python 3
Try to fix Sphinx error
Merge pull request #1812 from smichr/poly-coeffs

Poly coeffs
Merge pull request #1808 from smichr/symbols

3576: symbols handles multiple ranges
Merge branch 'master' into remove-old-wiki-refs