remove const from functions returning by value

[nglee]

Removed const qualifier on by-value return type.

From Scott Meyer:

Declaring by-value function return values const will prevent their being bound to rvalue references in C++0x. Because rvalue references are designed to help improve the efficiency of C++ code, it's important to take into account the interaction of const return values and the initialization of rvalue references when specifying function signatures.

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[alalek]

LGTM 👍

[nglee]

Test failed with PermissionError: [WinError 5] Access is denied. Is there a chance that this could be an error in CI?

[nglee]

Test failed with PermissionError: [WinError 5] Access is denied. Is there a chance that this could be an error in CI?

Tests passed :)

[rogday]

LGTM! 👍