import re
text = r'''Some text *here*:
* first item
* second item
* third item *some bold text*
and the end of text.'''
print(re.sub(
r'^[^\S\n]*\* |\*([^*\r\n]*)\*',
lambda x: f'**{x.group(1)}**' if x.group(1) else x.group(),
text,
flags=re.M)
)
aW1wb3J0IHJlCnRleHQgPSByJycnU29tZSB0ZXh0ICpoZXJlKjoKKiBmaXJzdCBpdGVtCiogc2Vjb25kIGl0ZW0KKiB0aGlyZCBpdGVtICpzb21lIGJvbGQgdGV4dCoKYW5kIHRoZSBlbmQgb2YgdGV4dC4nJycKcHJpbnQocmUuc3ViKAoJcideW15cU1xuXSpcKiB8XCooW14qXHJcbl0qKVwqJywKCWxhbWJkYSB4OiBmJyoqe3guZ3JvdXAoMSl9KionIGlmIHguZ3JvdXAoMSkgZWxzZSB4Lmdyb3VwKCksIAoJdGV4dCwgCglmbGFncz1yZS5NKQopCg==