requirements: remove sympy pin

[drewrisinger]

Sympy v1.5.1 is now available, fixes sympy/sympy#18056.

See https://github.com/sympy/sympy/releases/tag/sympy-1.5.1 & changelog: https://github.com/sympy/sympy/wiki/Release-Notes-for-1.5.1

Ref: #2648 #2655

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[drewrisinger]

@googlebot I signed it!

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[vtomole]

LGTM. Thanks!!