MAINT: Make python 3.9 the minimal compatible version

[bzah]

Pull Request to resolve #xxx

• Unit tests cover the changes.
• These changes were tested on real data.
• The relevant documentation has been added or updated.
• A short description of the changes has been added to doc/source/references/release_notes.rst.

Describe the changes you made

[github-actions]

Report

File	Stmts	Miss	Cover	Missing
icclim
icclim_exceptions.py	7	1	86%	20
icclim_logger.py	85	29	66%	24, 43, 65, 70–95, 105, 111–142, 150–155
main.py	278	26	91%	120, 129, 405, 490, 516–517, 519–520, 522, 524–525, 527–528, 547–554, 622–627, 708, 715, 719
utils.py	37	5	86%	22, 33–34, 36–37
icclim/ecad
xclim_binding.py	48	6	88%	39, 43, 68, 72, 97, 101
icclim/generic_indices
generic_indicators.py	482	85	82%	57, 60, 77, 81, 85, 98, 104, 178, 218, 225, 359, 388, 678, 709, 715, 723, 743, 906, 911, 931–937, 941, 971, 973, 1038–1039, 1072–1099, 1126–1147, 1163, 1172–1173, 1199–1229, 1235–1239, 1263
standard_variable.py	48	1	98%	19
threshold.py	402	21	95%	199, 219, 223, 271, 287–290, 305, 312, 388, 529, 598, 699, 902, 924, 930, 1004, 1007, 1106–1108
icclim/models
cf_calendar.py	35	1	97%	20
climate_variable.py	100	10	90%	103, 162, 173, 207, 213, 251, 255, 283–284, 321
frequency.py	176	11	94%	167–170, 314–316, 323, 401, 427, 432
index_group.py	34	1	97%	33
operator.py	29	1	97%	14
standard_index.py	28	3	89%	56, 59, 63
icclim/pre_processing
input_parsing.py	195	37	81%	74, 78–79, 84, 103, 167, 172, 178, 187, 200, 229, 245, 259, 279, 287, 289, 298, 307, 325, 339–347, 356–373, 395
rechunk.py	86	2	98%	38, 132
TOTAL	2569	240	91%

Test results

Tests	Skipped	Failures	Errors	Time
189	0 💤	0 ❌	0 🔥	1m 41s ⏱️