Cache SSL context in client.py

[evgkirov]

Description

Related issue: #279

Simple benchmark

# init_1000_times.py
from pyuploadcare import Uploadcare
for i in range(1000):
    Uploadcare(public_key="demopublickey", secret_key="demoprivatekey")

Before:

❯ hyperfine -N "poetry run python init_1000_times.py" --warmup=5
Benchmark 1: poetry run python init_1000_times.py
  Time (mean ± σ):      9.369 s ±  0.046 s    [User: 8.916 s, System: 0.285 s]
  Range (min … max):    9.322 s …  9.460 s    10 runs


~/Projects/pyuploadcare bugfix/279-cache-ssl-context* ≡ 2m 20s

After:

❯ hyperfine -N "poetry run python init_1000_times.py" --warmup=5
Benchmark 1: poetry run python init_1000_times.py
  Time (mean ± σ):     594.3 ms ±   4.0 ms    [User: 446.7 ms, System: 64.7 ms]
  Range (min … max):   588.6 ms … 600.6 ms    10 runs


~/Projects/pyuploadcare bugfix/279-cache-ssl-context* ≡ 9s

Checklist

• Tests (if applicable)
• Documentation (if applicable)
• Changelog stub (or use conventional commit messages)

[evgkirov]

Failing integration tests are not related to this change.