Unnecessary calls to sympy.N may increase computational cost

Description

Replace N(k) for k, where k = 1, 2, or 3

Motivation and Context

Improve performance. For instance
import sympy as sp

tic = time.time()
for k in range(1000):
a = 2
toc = time.time()
print('time without sympy.N', toc-tic, 's')

tic = time.time()
for k in range(1000):
a = sp.N(2)
toc = time.time()
print('time with sympy.N', toc-tic, 's')

outputs
time without sympy.N 5.888938903808594e-05 s
time with sympy.N 0.012852907180786133 s

How Has This Been Tested?

make test

Screenshots (if appropriate):

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.