Append to pythonpath instead of setting it

[angelhof]

@nvasilakis is there any way this resolves the installation issues?

Signed-off-by: Konstantinos Kallas konstantinos.kallas@hotmail.com

[github-actions]

OS:ubuntu-20.04
Sun Jun 19 00:42:52 UTC 2022
intro: 2/2 tests passed.
interface: 36/36 tests passed.
compiler: 54/54 tests passed.
agg: 109/109 tests passed.

[github-actions]

OS:ubuntu-18.04
Sun Jun 19 00:44:06 UTC 2022
intro: 2/2 tests passed.
interface: 36/36 tests passed.
compiler: 54/54 tests passed.
agg: 109/109 tests passed.

[github-actions]

OS = Debian 10
CPU = Intel(R) Core(TM) i7-8700 CPU @ 3.20GHz
Ram = 15752
Hash = 6e65175
Kernel= Linux 4.15.0-167-generic x86_64

benchmark	tests	passed	failed	untested	unresolved	unsupported	not_in_use	other_status
posix	494	375	41	31	6	40	1	0
intro	2	2	0	0	0	0	0	0
interface	36	36	0	0	0	0	0	0
compiler	54	54	0	0	0	0	0	0
aggregator	109	109	0	0	0	0	0	0

[nvasilakis]

This doesn't solve the installation issue it was created for, but I do believe it's a fix and should be merged.