polys: Faster initialization of PythonRational

Before:

In [1]: from sympy.polys.benchmarks.bench_solvers import *

In [2]: %time time_solve_lin_sys_165x165()
CPU times: user 16.76 s, sys: 0.08 s, total: 16.84 s
Wall time: 16.80 s

After:

In [1]: from sympy.polys.benchmarks.bench_solvers import *

In [2]: %time time_solve_lin_sys_165x165()
CPU times: user 8.55 s, sys: 0.02 s, total: 8.56 s
Wall time: 8.56 s