MAINT: Remove reference to old 2.0.0 branch #4377

Job	Run time
Build & verify package	14s
pytest on windows	6m 42s
Check code style issues	46s
pytest on 3.7 (crypto-lib: cryptography)	3m 43s
pytest on 3.8 (crypto-lib: cryptography)	4m 51s
pytest on 3.9 (crypto-lib: cryptography)	3m 41s
pytest on 3.10 (crypto-lib: cryptography)	3m 47s
pytest on 3.11 (crypto-lib: cryptography)	3m 55s
pytest on 3.12 (crypto-lib: cryptography)	4m 57s
pytest on 3.7 (crypto-lib: pycryptodome)	3m 39s
pytest on 3.7 (crypto-lib: none)	3m 40s
Combine & check coverage.	21s
	40m 16s

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