patch_qmake_qt_key: Fix some warnings reported by pylint

 W: 47,0: Unused import shutil
 W: 49,0: Unused import datetime
 W: 55,33:replace_key: Redefining name 'key' from outer scope (line 138)
 C: 70,4:replace_key: Invalid name "f" for type variable (should match [a-z_][a-z0-9_]{2,30}$)
 C: 78,4:replace_key: Invalid name "m" for type variable (should match [a-z_][a-z0-9_]{2,30}$)
 W: 84,4:replace_key: Unused variable 'remaining_max_length'
 W: 98,31:fetch_key: Redefining name 'key' from outer scope (line 138)
 C:106,4:fetch_key: Invalid name "f" for type variable (should match [a-z_][a-z0-9_]{2,30}$)
 C:117,4:fetch_key: Invalid name "m" for type variable (should match [a-z_][a-z0-9_]{2,30}$)
 C:138,4: Invalid name "key" for type constant (should match (([A-Z_][A-Z0-9_]*)|(__.*__))$)

Change-Id: I5bcd23d32eaea4eef6bbffcff06f3f5a14006760
Reviewed-by: Iikka Eklund <iikka.eklund@digia.com>