Made MAX_EVAL_SECS configurable via environment

[ecmascriptguru]

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What does this PR do?

• MAX_EVAL_SECS value hardcoded as 10 seconds and I wanted to get it increased but no way. So made it configurable.

Where should the reviewer start?

• Simple. Just check the decorators.py

How should this PR be tested?

• No big change but anyone can configure the timeout raised by stopit.

Any background context you want to provide?

• No background.

What are the relevant issues?

• OPTIMIZATION PROCESS STOPS : stopit.utils.TimeoutException #1088
• [Question] TPOT can't run for a lot of time #1200

Screenshots (if appropriate)

• No screenshot

Questions:

• Do the docs need to be updated? Maybe yes.
• Does this PR add new (Python) dependencies? No.