chore: change runtime python version to 3.8

Stop supporting Python 3.7

main.py

@@ -1,3 +1,6 @@
 import okama as ok
+from importlib import metadata
 
-print(ok.__version__)
+print(metadata.version('okama'))
+
+# print(ok.__version__)

okama/__init__.py

@@ -19,6 +19,8 @@
 - Matplotlib visualization scripts for the Efficient Frontier, Transition map and assets risk / return performance
 
 """
+from importlib.metadata import version
+
 from okama.asset import Asset
 from okama.asset_list import AssetList
 from okama.portfolio import Portfolio
@@ -37,4 +39,4 @@
 from okama.common.helpers.helpers import Float, Frame, Rebalance, Date
 import okama.settings
 
-__version__ = "1.2.2"
+__version__ = version("okama")