Fixes missing known constant in pycode.py
Fixes missing known constant in pycode.py

Before this fix the if one tried to lambdify an expression containing
`E` one got the following error:

```
>>> fun = lambdify(x, E)
Traceback (most recent call last):
 File "<console>", line 1, in <module>
 File "/home/user/github/sympy/sympy/utilities/lambdify.py", line 754, in lambdify
  funcstr = funcprinter.doprint(funcname, args, expr)
 File "/home/user/github/sympy/sympy/utilities/lambdify.py", line 996, in doprint
  funcbody.append('return ({})'.format(self._exprrepr(expr)))
 File "/home/user/github/sympy/sympy/printing/codeprinter.py", line 107, in doprint
  lines = self._print(expr).splitlines()
 File "/home/user/github/sympy/sympy/printing/printer.py", line 287, in _print
  return getattr(self, printmethod)(expr, **kwargs)
 File "/home/user/github/sympy/sympy/printing/pycode.py", line 74, in _print_known_const
  known = self.known_constants[expr.__class__.__name__]
KeyError: 'Exp1'
```

current commit fixes this error, and instead gives the expected output:

```
>>> fun = lambdify(x, E)
>>> fun(23) - exp(1.0)
0
```

kaushik94/sympy,kaushik94/sympy,kaushik94/sympy